Both the Strobist and Rui talk about the virtues of using gels on strobes to create atmosphere in a shot. Either cold blue or hot read. Uber photographer David Tejada uses gels on a regular basis to spice up his shots.
Reader Tony Bell has an interesting idea on color correction gels. Even though they are cheap and available, you can still beat the price, if you are going for Lomo style and Lomo level color accuracy.