I ran across Retrievr earlier today, and found it to be very interesting, if not extremely similar to a rumor about some Google image searching capabilities from a while back. Still, it’s surely something to play with and see what you can find.
Essentially, you go to the Retrievr page, and you will see an empty box above a color palette. Click and move your mouse around in that box, and you are creating a little bit of art. Now, try something very simple. Say, for instance, a single horizontal line. Then, when you stop, the page will load all the contributed flicker images that match that have a similar pattern slope involved in them. Changing your color will change the results.
It’s honestly easier to see it and try it out than it is to describe it. You’ll see exactly what it does with one or two tries. Definitely fun to toy around with and people have contributed some amazing art.