Heard about this on 1up Yours. It matches you up with another user at random, shows you both an image, and you suggest tags for it until you make a match. The more specific your matching suggestion, the more points it's worth. Yep, you're basically doing the work of tagging the images for them for free, but it's actually pretty fun.
Oh, and there are leaderboards for highest-scoring session of the day and all-time total score. (I'm lucky to get over 800 points a session, I'm shocked somebody's accumulated over 15 mil).

