TinEye is a reverse image search engine
. Upload or link to a photo, and it will tell you where that photo shows up on the net. Handy to see if someone is using your artwork, or if an artist is putting other people's artwork on his page... you get the idea.
They're only in beta, so results are spotty. But the engine is smart enough to spot images when they're surrounded by text, and I imagine they'll be indexing more and more pictures as they get up to speed.