If you haven't seen this yet, go and check it out.
What it does is basically allows you to either upload an image or image URL, and it will show you all the other websites on the net that it knows about that contain the same image.
But the really amazing part is that the images from other sites don't need to be identical - it can also find images that are derivative works from the image you tell it to search for. For example, search for this image and see what happens:
http://farm2.static.flickr.com/1095/...bb857f6bd7.jpg
Amazing.
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