Retrievr is a neat Flickr hack from System One that searches a database of images based on simple drawings rather than tags. I drew the branch on the left and the image on the right was one of the search results. From the project description:
Does it actually work?
Yes! That is, it depends. (Mainly on your expectations!)
In my experience, the results are usually fairly good, sometimes even stunning – considering the artistic sophistication most of us are able to come up with (gallery forthcoming); and in the cases they're not so stellar, they are at least entertaining ;-) But clearly, the approach has its limits.
One thing to keep in mind is that retrievr doesn't do object/face/text recognition of any kind, so if you're drawing an outline sketch of a chair, it almost certainly won't get you one back (except your index only contains images of chairs). The same holds for corporate logos, icons &c.
It helps to think of it as matching the most pronounced shapes and slabs of colors.
Link to Retrievr, Link to info about other "Search By Drawing" projects (via Drawn!)