Luis von Ahn is a GENIUS!!!
He developed a way to make people label images, mark regions to locate objects in images and is working on more stuff - by making people doing it for him. And they do it for free. And they enjoy it. And they continue to come for more....
ESP Game is a game where two people have to label given images. Once they both give the same label for an image, they get scores. The result is a pretty accurate set of labels for the images.
Peekaboom - gives the first user an image and a label. He then must point to areas of the image that represent the label. The second player is shown only the area around the points marked by the first player, and must guess the label based only on the portion of the image that he can see. The result is that the labels are given pretty accurate locations inside the images!
The basic idea of transforming difficult tasks, such as computer vision problems and the like, into games is what he calls Human Computation, and its potential is huge! Porn sites use it to resolve CAPTCHA in a very straight-forward way. The gaming approach is essentially the same, except that it's more moral, it's fun, and could be adapted to a lot of other very difficult problems.