Google has launched a clone of the ESP Game with the rather dull name Google Image Labeler.
Google Image Labeler is a simple game where two anonymous, random people try to agree on labels for images. The idea here is to get people to do useful work, such as providing accurate labels for images, by trying to make it into a fun game.
See also my earlier post, "Human computation and playing games", where I talk about ESP Game and Luis von Ahn's other work in more detail.
See also Luis von Ahn's talk at Google on Google Video, "Human Computation". It is a light, fun, and very interesting talk that includes demos of these types of games and the value of the data derived from the games.
[Found via Philipp Lenssen]
Update: Danny Sullivan reports that Google licensed the ESP Game from Luis von Ahn.
Update: A Google engEDU talk on Google Video, "Using Statistics to Search and Annotate Pictures", by UCSD Professor Nuno Vasconcelos gives some nice examples of how this kind of image labeling data can be useful for image search.
Update: A couple other talks are worth considering here as well. First, a talk "Detecting and Recognizing Objects In Natural Images" might be another in a long line of applying probabilistic models to finding objects in images, but I particularly enjoyed the demo at 15:00 of their text recognition in that talk. Second, I thought "Automated Reconstruction of 3D Models" had some impressive ideas about how to quickly get detailed 3D city models into mapping software like Google Maps and Google Earth.