Ed Felten tells us (well, me, anyway) about a game created at Carnegie Mellon University called The ESP Game:

A group at Carnegie Mellon University has created The ESP Game, in which a pair of strangers, shown a photographic image, are each asked to guess the single word that the other will use to characterize the image....The brilliant part is that the game “tricks” its players into doing an important and incredibly time-consuming job. By playing the game, you’re helping to build a giant index that associates each image on the internet with a set of words that describe it.