fastr is a very cool game, not unlike Guess-the-google. It shows you 10 images (from flickr) that have a common tag. You’re supposed to guess the tag before all 10 images appear. The faster you guess the tag, the more points you get.

The nice thing about this game is that it is round-based (all online users of the game play at the same time) so you can see how other users are doing with the same challenges and compete with them.