A fascinating, though kind of odd and experimental site, is Retrievr. It lets you do a rudimentary paintbox style sketch on the computer, then tries to match up the sketch to an image from the massive Flickr photo database.
For instance, this sketch:
Produced hits including these:
It would be interesting to know what technology goes into matching this, but otherwise, it’s just one of those interesting things. It did strike me as a possibly useful tool for artists looking for photo references (especially for comics?), but I’d say it needs more refining.