An interesting idea from Google: they want to improve the quality of their knowledge about online images, so they create a game, the "Image Labeler". Could be difficult to play when people have serious spelling issues, but could be fun for others ..
Essentially the program matches you up with another person (you may play as a guest), and you both add words to describe each image, until you come up with a match. This gives the search engine more information about the pictures it has found. Matching two strangers is a way to ensure they get the right information, by drawing on people's competitiveness. Clever.