Batman and the Joker. Harry Potter and Lucius Malfoy. The Powderpuff Girls and Mojo Jojo. Throwing a villains and heroes party is a fun way to add a theme to a generic costume party, and works for almost any age group. Invite your guests to come dressed as their alter-ego: a favorite villain or hero from the movies, TV or books (including comics).
Decorate in black and white, classic good and evil colors, and play games where heroes battle villains -- anything from a simple kids game of tag to a round of My Marvel Heroes Monopoly.
Extra points given for the perfect evil villain cackle.