Tool for searching images

I've been looking for a tool that would search for picture's origins in the internet. If I have a picture on my computer and want to find out where is this picture from. What could I do, except asking ppl or trying to guess keywords in google? Is there a tool which could help me?

If we search only unchanged pictures, then saving picture hashes and urls and searching by hashes would work perfectly. If we want to find slightly modified pictures as well (like saved .jpg->.gif or changed size), color histograms could be made of parts of image and then compared on similarity. While it's not character recognition system, it's not so difficult to do. Therefore I assume someone has already made the tool. Then, where is it? There HAS to be one!