A new search engine, TinEye, is out that's dedicated to finding images on the web. You upload your picture to their server, and on the basis of its image analysis—its "digital fingerprint"—the software goes looking for matches all over the web. They have crawled about a half billion images to date, and are looking to extend that considerably in the future.
The visual matching system can find cropped elements of a picture, and even ones that have been creatively "mixed." Plus, the site gives you an immediate visual comparison between your original search target image and what they have found.
First use for me: tracking down visual elements that I have downloaded but not really properly notated. What a great way to find authors...and abusers.