Hair is where the horror is.
Medusa
Fake snakes, bobby pins, and a few easy braids and BAM, you're Medusa. Bonus: The messier this hair looks, the better. Click here for instructions.
The Joy of Fashion / thejoyoffashionblog.com
Spider
Finally, a use for all of those friggin' pipe cleaners you've had sitting around for years! Get the instructions here.
Simple As That / simpleasthatblog.com
Mouse
Follow these directions to make little mouse ears, and you're all set.
Gleeful Things / gleefulthings.com
Butterfly
Braiding skills, will power, and a load of hairspray may be required, but there's no way your friends won't be impressed when you show up as a butterfly. There's a full tutorial here.
All Day Chic / alldaychic.com