Update 2011: Google has finally published its similar image search engine, as an alternative for text search, google images can now search with an uploaded image, and from the image’s url.
give an image to TinEye and the search engine tells you where that image (or a similar one) has been used on the web. The TinEye similar image is a reverse search engine for images, is pretty cool and fast is you need an image comparison search engine for your copyright needs and for the moment is free (lets you find unlimited similar images).
TinEys is the perfect image search engine to find image stealing. This website is also very good to find photoshopped versions of original photos, since it can compare even small parts of the original picture inside photo manipulated pics.
The TinEye image search also features browser plugins to skip the image upload step.
here you find the video to some features related to TinEye commercial use. TinyEye has been tested by our staff with goatse images, and it works!
Update: also google is developing a new similar image search on google labs.