The Grudge. Or the girl from The Ring (Sadoko or Samara depending on where you're from). It's a classic costume, definitely spooky, and it's fun to act like a creepy girl! The costumes are cheap, an inexpensive white night gown, fake blood, and a cheap Wal Mart wig. Mess the wig up, make it look tangled and style it to cover you face. Cover the nightgown in fake blood, and badabing badaboom you're set. Shoes can be an old pair of slippers or anything white or black.
Last year I just wore a name tag that say said "hello my name is maybe" and this year I'm going as my idol. I've seen some people go as giant Telly tubbies, an iPod, bedside table, penguins, celebrities, a smoke detector, and zombie ballerina.. Really depends on your age and how much you want to spend and how much effort you want to put into it.