Retrievr is a web-toy that lets you search images on Flickr by drawing things that look like what you're looking for on a Flash applet. It seems to go mostly by colour, rather than feature recognition, and seems to only search a small pre-cached subset of images (which tends towards the prettier end of the scale and includes a lot of sunsets, cats and arty black-and-white photos). As far as finding pictures of
objects one draws, it's rather unsuccessful; though, nonetheless, it does yield interesting results.