There are many commercial costumes available at the local stores, however between the cost, and lack of imagination, and the fact you end up with a half dozen kids all looking the same at a party. Nothing beats homemade. Below is some basic ideas for you to play with. All are cheap, easy, and allow for tons of imagination. By the way if you have an idea to add please feel free to send it in.

The old stand by a Ghost: Grab a old white sheet, cut out the eyes. Be sure its is short enough not to trip you.  Take scrap piece of sheet, and sew up a simple treat bag.
OR 
as  Kathryn suggested:
Why be the same old plain ghost?  A ghost going out in public might like to get dressed up a little.  Use the same old white sheet, cut out holes for the eyes and a small slit for the mouth.  Then dress it up a little.  Put eye shadow and lashes around the eyes.  Lipstick for the mouth.  Put a little color on the cheeks.  How about earrings and a necklace ?   You could color in a beard or mustache.   Use something besides just a white sheet to be a little more fancy
 

Clown: Sweet clothes that are too large. Decorate by using either fabric paint, or markers. For those who dare try tie-die.  Tack on some large pop-poms, add a cheap wig, hat, and make-up. You may also stuff the extra space in clothing with tissue. However this could be messy, if you need to make a "pit-stop".

Scarecrow:  Over sized flannel shirt, faded blue jeans with patches, rope for belt and to tie at the cuffs of his jeans and sleeves, stuff the shirt and jeans with plastic grocery, or paper, a little bit of straw to hang out of jeans and shirt. Add a little bit of make-up. 

The American tourist: Guys loud Hawaii shirt, shorts (if weather permits), Large straw hat. Gals bright loud dress, flats, big sun hat, straw bag. Don't forget to have a camera, and maps hanging out, and that look of being lost. Guys can use a tourist bag, (like you get from travel agents, and gals can use the straw bag for their treats.
 

1960s Hippy: This one is hard, after all its all back in style! What is real, and what is costume? ;-)  Tie die clothing, long wig (in the old days we called a "fall", sandals, headband, anything with fringe on it (vests, purse...) light colored sun glasses.

Old man: Dark paints (cuffed), and jacket, most any dress shirt, mans flannel hat, walking cane. Gray the hair, with any of the commercial temporary colors, or dust with a little flour.

Old woman: Old style dress, just below knee, knee-hi stockings rolled down a little, oxfords, large black purse.

Your chance to be a real nerd. Slick back the hair, grab some paints that are too short, white shirt, white socks, penny loafers, or lace up oxfords, pocket protector, with lots of pens, dark rim glasses (don't forget to tape the corner, or at least paper clip it) briefcase, or backpack for treat bag.

From our guests:

Japanese Doll: My daughter won with this costume! Her face was painted white, she had red rosy cheeeks all shaped in a circle, Hair in buns, thoung slippers, and a kimono. From Lani S.

Miss. America
Use a old bridesmaid dress of formal, add a pair of gloves, a tiara, some oversized jewelry, and heals. For the banner, a piece of wide ribbon, and write "Miss. America" in glitter. From Jacque

Soda pop can: Use a round barrel, cut a hole in the bottom.  You cut holes for  arms and your head.  Paint it the colors of a pop can.  Presto!  You have a pop can!!!  - from Ann

Static Cling
Wear anything a solid color is good all white or all black.  With safety 
pins pin anything to yourself i:e: socks, underware, bounce sheets, etc. You can also spray your hair straight up in the air. From: Maria

1950’s “Sock-Hopper”
It’s quite simple, just a round skirt, a white short-sleeved shirt, and some saddle shoes.  (If you don’t have saddle shoes, plain buckle shoes are fine.) You could add some cat-eye glasses if you want.  Tie their hair in a ponytail, and wrap a scarf around it. From: Ziemniak

Mummy
Wrap yourself in white bandages (rolls of gauze)  Use splotches of green paint for "mold".  Use white make-up on face with "age lines" drawn on.  The stiff-legged gait works perfectly. From: Chris

Quasimodo the Hunchback of Notre Dame
Build the "hunchback" out of foam or cardboard.  Find an old coat several sizes too large to accomodate this.  Old, ratty clothes are great!  Wear a snaggly wig and keep putting hand up to ear and muttering "eh?". From: Chris

Pirate
Form a fake aooden leg by wrapping your bad one in brown felt.  Wear sailor pants or jeans with a striped top.  Tie a bandana around your head, wear an eye patch, carry an empty bottle marked XXX (for the "rum") and mutter "aargh" a lot.  Ask for "Trick or treat, matey!" From: Chris

Crash test dummy - After!  There is a commercial costume for this, but a good one can be made with sweats and tape to make the markings on it like the crash test dummies.  Use bandages and fake blood to decorate. Fashion a seat belt out of webbing and an old buckle and pin in place.  (or go to a junk yard as ask for an old seatbelt). From: Chris

Train Engineer
 A pair of blue jeans, a blue Jean jacket, a red  and black bandana. a blue and white engineer hat, a whistle, and play lantern   From: Brenda

Mother Nature

• Silk flowers and silk plant leaves in a variety of shapes and colours an old skirt and vest 
• a face mask
• € twigs (collected from your nearby park or in your own backyard) to make a nest
• € a foam bird that you can find at any craft store
• € glue gun and wire

Skunk
Black with white strip down the pack.. be careful everyone may run

Bunny
Pink, brown, or white, add a powder puff or large pom-pom to the backside, and a set of ears make from constriction paper and a plastic headband. Now just hop on out and enjoy

Turtle
Green with a piece of poster board attached to the back. Simple cut the poster board into a large oval, make random marks like a turtle shell, and paint.

Puppy

Brown, tan or white, either paint, or pin paper spots on. Make floppy ears and bobby pin to hair over your ears. The tale can be most anything, depending on what type of doggie you wish to be. Add a little make-up.

Kitten
Gold, brown, tan, white. Make ears from construction paper, attach to head band or bobby pin in to hair. Sew a simple round tale, and pin on.

Spider
Black,  For the legs grab some cheap black hose, stuff with any thing dark and light weight. Either sew, or staple to keep stuffing from falling out. Pin to  clothing, add a little make-up

From our guests:

Lady Bug
Use a pair of black sweats and black sweat shirt (and a pair of black shoes). Use a large piece of poster board or cardboard and cut out a large oval. Paint it red with black dots. Staple black elastic pieces (in a loop) onto the board to make arm holes. For the antenna atatch craft pipe cleaners with black pom poms on the ends to a headband. From: Shaundi Gopp

Snowman
Last year my 3 year old was a snowman. It was the easiest costume so far! I bought white sweats, christmas socks, white keds tennis shoes, a cheap black hat, fake snow, mittens, scarf and black pom poms for the coal! 
I sprayed the hat to look like snow and added a cheap red bird from the craft store. I bought a white mask and bought a cheap carrot from the craft store and glued it onto the nose part of the mask. I glued the pompoms on his sweatshirt and that was it. Cheap, simple and he was adorable. Besides he was the only frosty we saw! He also had to have white face makeup on!!   From: Andee Heneghan

Cereal Killer
Attach little cereal boxes to a shirt, cut a slit in the boxes, glue in plastic knives into the holes, drip red paint running down from the knife hole in the box. From:Gail McNitt

A Bunch of Grapes 
Using green or pruple sweets, attach balloons of the same color with safety pins.  Ppaint face and hands same color. From: Barbara Brown

Grapes appear to be popular! Here is one from Jo:
My fav was purple tights, purple (long sleeves) body suit (I can't think of what it's really called), 12 purple balloons and a green felt hat. Balloons are pinned (by the tied off end to the fabric in a triangle shape, 3 on top, 2 in middle and 1 at bottom (on front and back).  I was a bunch of grapes and the hit of the party!

Bag of Jelly Beans
- lots of different colored balloons
- large clear plastic bag (I got mine from the grocery store - the BIG bags that they put all the recycled cans in)
 - colorful ribbon
- two safety pins
- two pieces of white cardboard

Blow up all the balloons. Cut two leg holes in the bottom of the bag and two arm holes near the middle. Have your child stand in the bag and fill with blown up balloons. Write on each piece of cardboard "JELLY BEANS 2 CENTS EACH". Safety pins signs to front and back of bag. LOOSELY tie around shoulders with a colorful ribbon. Cut off excess bag around face From: Dee
OR
 If your prefer to be a little high class Jelly Bean... Natalie suggested...put an enlagred Jelly Belly sign on the front of the bag.
 

Black Eye Pea
When I was younger, a teacher from my elementary school dressed up as a "black eyed pea" and it was so cute, my mother dressed me up the same way the next year.  It is simple and cheap.  Wearing all white, with black tennis shoes, I cut out the letter 'P' from black construction paper and pinned the P's all over the white outfit.  I painted a black spot over one eye ("black eye" ... get it? :) hee hee) and I was a "black-eyed 'P' "  From: Angela

Black Bat
Using a black umbrella and tights, Rip the umbrella in half. Discard the center and innards of the umbrella. If you are fancy you can sew the edges of the umbrella  to the back of the body. If you don't have the time or patients to sew the umbrella you  can always safety pin it. From: Sydhen
 

Butterfly
Put on black sweats and black shoes. Use a piece of poster board and paint wanted color cut them out in an appropriate wing shape. punch two holes toward the edges of the wings. put black ribbon through them and tie around waist. attach pipe cleaners and pompoms to a plastic head band.  From: Bekjg

Blooming Idiot!
Get a green body suit and some green tights and on your head wear a beanie hat. Glue fake flowers to the beanie hat and maybe pin a few on your body suit. Also it's fun if you paint some flowers on your face and arms. I found some cheap, fake Ivy and wrapped it around my neck and arms and presto! Your a
blooming idiot!  From: Kym Tucker

Spider to Bat
You need an oversized black sweatshirt, black sweat pants,  black open faced ski mask, two HUGE googly eyes, black yarn or fishing line, pipe insulation (for legs!), three pairs of gloves (very small) and black opaque tights.
Attached the ski mask to the neck of the sweatshirt.  Hot glue googly eyes over the opening in the ski mask,  The foam pipe insulation is placed into tights.  So that the "leg" could bend, I cut the length of pipe insulation in half.  Attach all legs together for coordinated movement with black yarn or (fishing line),  Attach toddlers gloves on the ends of the "legs" and have your child wear one pair.  For safety, I bought some reflective tape and created a "design" on the back and front so that he could be seen.  Obviously, wear black sweat pants and dark shoes.  Two years later, we took the legs off, added a huge set of black felt wings hot glued from wrist to neckline to wrist and used craft foam to make big black bat ears!!   From: Amy

Darcy Prairiedog (Little Dogs on the Prairie) 
Items needed: Light and dark pink felt , an old hooded pink sweatcoat, pink gloves, pink slacks, pink slippers.
Cut out light pink felt into what looks like prairiedog ears. Place the dark pink felt inside in in the same shape except smaller. Glue onto the sides of the hood on the sweatcoat. With the remaining felt,cut out a piece of "headfur". From the light pink felt. Put on and paint your face pink with a black nose. From: Patricia 

Ragdoll
One year for my girls I had them dress up as ragdolls and I didnt have to spend a penny!
I took an old sleeper and cut out the feet then I sewed patches in various places over the sleeper. I also sewed wide cuffs at wrist and ankles. (because the sleepers were to small). It looked so cute. For the head I used an old stocking cap and sewed strips of fabric from old clothes to the stocking cap. Then gave them red cheeks and freckles. It was a great costume and completely free. From: Evelyn

Basket of laundry...clean or dirty. 
Wear sweat pants and sweatshirt if it is cold. Tights and a turtleneck if you will be in a warm building. Take an inexpensive (thin) plastic laundry basket and cut a hole in the bottom that you can fit through.  Make it small enough so that is "sits" on your hips after you pull it over your head.  Fill basket with laundry and pin a few dryer softener sheets to your sweatshirt.From: Dede

Bubble Bath
Take a white laundry basket and cut out the bottom, be sure to tape the cut edges with masking tape. Attach two wide strips of material (white) to front and back of basket (these are the shoulder straps).  Attach white, light peach, and pearl balloons to the staps using safety pins (these are bubbles)  Wear a white bodysuit, and white pantyhose.  Put on your houseshoes, shower cap, and grab your scrub brush.  (If the laundry basket has holes in it, cover the inside with posterboard)  From: Bridgette

Just use your imagination. 

Pokemon card
Last year I made my 6 year old into his favorite POKEMON card.  I used a box and made his head BLASTOISE'S head.  My computer scanner, felt ,my computer's publisher and a little creativity made him the hit of the party! - from Bonnie

Paint it silver, glue on a few extras a presto.. a robot.

Paint it red, tack on some paper plate wheels, and you are a race car.
 

Paint it white and a handle and you will be a cool refrigerator.

Paint it light tan, tack a small flat box. Draw on a keyboard, you will be the first walking Macintosh. (may work with Windows ;-)  )

Wrap it in Christmas paper, add a large bow.. wow a Christmas present.

Paint it bright colors, and a crank. You can pop into the local Halloween party as a Jack-in-the-box

Cover with table cloth, glue on silver plastic platter in the middle (cut hole for head to pop through. Glue on a couple of place settings of Plastic dishes, and glasses. Now your head in on a silver platter.
 

Thomas the Engine & Friends
Get a small box big enough to fit around your childs waist.  Paint box
 blue (for Thomas), or green (for Percy) etc. With colored construction paper add stripes and train number to sides, also add black train wheels.  On front make a 3D face with construction paper to match your train of choice. Attach a belt from a robe to each side and harness over shoulders. Dress child up with a conductors hat. 
From: Trish

Rubics cube
 Using a large card board box and five different colors of contact paper  or construction  paper (contact paper is easier to stick on)  enough to do 5 sides, black pants and long sleeve shirt. this was done very inexpensively and we had a lot of fun with it. From: Tracy 

Dice
Grab a friend and take 2 cardboard boxes, paint them white.  Cut out several black circles (From construction paper) and glue them in the appropriate places to make you and your friend a pair of dice!  It's so easy, From: Laura Jones 
OR
Have the second person dress as a key ring  and use a light weight chain to attach the two people. To make the key ring cut several large keys out of poster board, attach one to the back and another to the front. Attach a medal ring to a hat, then attach a few more keys  to the  ring. From: Joy, Codi & Sarah

Lego Block
Take a medium sized box (apple/orange boxes were a good size) cut two round holes in the sides for the arms.  Hot glue 6 cottage cheese containers on the top of the box (the flat part)  like a Lego piece.  Spray paint the whole thing whatever color lego you want to be.  (We chose our colors based on what color sweats and turtle necks we had on hand) Take a smaller box that will fit on top of their head snugly and hot glue one cottage cheese container and spray paint it the same color.  Just slip into the open part of the back of the box and stick your arms through the holes. From: Elliott and Danielle and Kevin and Angela and Kristie (this one is real popular this year!)

Dresser!
Walmart or Kmart sells cardboard dressers in the laundry department. Cut out holes for arms on the sides and hole on top
for head.  I put foam in the inside for the shoulders to hold it up.  I painted the dresser and wrote on each drawer underwear, socks, ect. Hot glue a doily to top with hairbrush and hair accessories.  Hang underwear, socks, shirt, and pants out of drawers.  Put a small lamp shade on child's head so neck and face are base of lamp. From: anonymous (by way if you don't want to spend the $$ for a cardboard dresser, it wouldn't take much to make one out of a box. Attaching long flat boxes on the front and hot glue small balls on the front for pulls.
 

With a little thought you may come up lots of ideas that make the chair a natural part of the costume. Here are a few ideas to get you started.

King or Queen on Throne

Convert chair into a throne, by wrapping the back and arm rests with gold  or silver foil / paper.  Use plastic gems (from craft store), old jewelry, Christmas garland to give it that "special look".

Simply wear a cape of a royal color (velvet looks great). Scepters and crowns can be purchased at most retail stores, or make your own! 

If an attendant will be going, they may dress as the mate (King or Queen) or as a court jester.

A variation of this would be to dress like Santa and have the attendant be an elf.

Skier

Skiers have a bad habit of falling and breaking a bone or two. This opens the door for a great costume.  Just put on some ski clothing (hat, scarf,...) Wrap a leg in wide white gauze (to look like cast) and if you really want to "ham it up" and you use a power chair, wrap an arm too.
 

Ventriloquist
If you know anyone who has a ventriloquist doll, this can be a quick and easy one. You may wish to go with the formal look wearing a tux, or the casual look by dressing to match the doll.

Gift,  Jack in a box, or Robot

Find a box that will fit conformably over  chair (still allowing you room to maneuver inside).  Remove top flaps, cut hole in top for head, and in sides for arms. Decorate.

For gift box, wrap box in wrapping paper, make a large bow and wear as hat.

To be a Jack-in-the-box, paint the box in bright colors, than wear a matching top and hat. Clown make-up will add that little extra.

To be a robot, cover box in foil, then use your imagination to decorate. 

Car, Truck, Space Ship

Again using a box (as above) decorate as you favorite vehicle. 

The space ship can be done much as the robot. This can be real cool costume for high tech power chairs and or using a vent.

Make Your Own Make-up
You will need:
           2 tablespoons of soft shortening
           4 tablespoons of cornstarch
           Food coloring 

Mix ingredients shortening and cornstarch. Separate the recipe into different cups, add food coloring.

 Wash your face and any area that you will be applying makeup to and than dry thoroughly.

Apply a thin coat of cold cream to the entire face.

Using a cotton ball, gently pat cornstarch over cold cream. Keep your eyes closed.

Apply make-up with fingers tips.
 

Others web sites with great ideas:

Berkely Parents Network - A pretty cool list of other costume ideas.

Halloween Express - Lot's of unique costumes, ready made

Family Prod - While the text of this site is written in French, it offers some great ideas. One of the great things about pictures.. a picture is worth a 1,000 words. ;-)

Halloween Costumes, made simple