TinEye

From Idee:

Traditionally, Web search has relied on words or queries to scan massive indexes of pages for results. Searching for images can be a little trickier though. You're often relying on the competence of whoever uploaded the shot to provide the proper file name--and in a very small percentage of cases extra hints in the metadata to help the search tool get its hooks in. The tool works best with popular or otherwise well-known images. Nearly everything else I tried didn't produce much. Some of the searches with the most results have been compiled in a 'cool searches' section, but typically feature well-known art or photography. The company expects to have a better set of results as its index increases in size. One of the more interesting uses for this technology is tracking down stolen intellectual property.