The ESP Game
This is pretty interesting. What happens is, you and a strange who you cannot communicate with log into the ESP server, and you both take a look at the same image. You then both have to guess a word that is associated with the picture. Most pictures already have some words associated with them, so you have to pick something new or different. When both of you have guessed a word you move onto the next picture.
Of course this is a way of getting people instead of computers to categorize pictures on the Internet but it's still a lot of fun. Check it out.
Link (via Monkey Methods)