According to The National Retail Federation’s (NRF) 2008 Halloween Consumer Intentions and Actions Survey, the top Halloween costumes for children in the United States are:

Princess (generic)
Witch
Hannah Montana
Spider-Man
Pirate
Star Wars Character
Pumpkin / Jack-o'-lantern
Batman
Disney Princess
Ghost (tie)
Athlete / Sports figure (tie)

The top Halloween costumes for adults in the same year were:

Witch
Pirate
Vampire
Cat
Fairy (#5 tie)
Nurse (#5 tie)
Batman (#6 tie)
Political figure (#6 tie)
Ghost
Angel
Clown (#9 tie)
Wench/vixen (#9 tie)
Athlete (#10 tie)
French Maid (#10 tie)
Queen (#10 tie)