TinEye –is a site that performs a reverse search on an image. Give it an image (upload a file or point it at a URL) and then TinEye will find other places on the web that contain that image.
Besides the novelty, more practically people often use TinEye to detect content theft - when their images are being appropriated and being displayed with incorrect attribution.
http://www.tineye.com/
Although the specific algorithm TinEye uses is not known (the code is not published), Neal Krawetz recently posted on the Hacker Factor blog elaborating his take on how this might be done via perceptual hash algorithms.
http://www.hackerfactor.com/blog/index.php?/archives/432-Looks-Like-It.html
