Update (2013): the site is no longer available.
Google Image Labeler is a place where you can help Google improve Google Image Search by tagging images.
It's similar to what you do in Google Video. But Google transformed this boring activity into a nice interactive game. Everytime you login, Google finds you a partner and shows you the same images for 90 seconds. The first label found by both of you is considered a good answer and brings you 100 points (while two people can both fail in finding a good label, the probability is much smaller than for one person). After the time expires, you can see where the images came from and what were your partner's labels. And then you can start again, as the game is addictive.
The same images will be shown to other people, so, in the end, the images will keep only the best labels. This way, when you search for "car", you'll only find pictures of cars. I wonder if Google intends to use the labels directly or it's just a test for their recently acquired object recognition system.
A similar game is The ESP Game and this was the inspiration for Google Image Labeler. Here are the rules of the ESP Game:
* two-player online game
* partners don't know each other
* partners can't communicate
* the goal: type the same word
* only thing in common: an image