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?Link to Retrievr, Link to info about other "Search By Drawing" projects (via Drawn!)
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.