Have you tried the Google Image Labeler? It is a game where you and a randomly chosen partner are shown images one at time. You and your partner have to type in possible labels for the image. You keep typing labels until you and your partner have a label in common which earns you both points. Then a new image appears and you start again. This goes on for 1 minute and 30 seconds. After the round is over, you can choose to see the labels that your partner typed in.
Once I started playing, I got hooked and played for about half an hour. It is kind of fun and nice to know that you're doing your part to help Google better classify its images.