A visual search engine that returns images of a similar look and feel. I’ve found it to be surprisingly effective: put in an image of a very clearly defined object and you’ll get lots more of them in return, but put in something a little more abstract or metaphorical and the results can be really good. The only issue I can see is the same one that always crops up with anything related to image search, and that’s right and reuse—Same Energy only provides a link to the source (which might not be the original source anyway) when you sign up for an account, and makes no attempt to bring through rights information.
See also:
TinEye, a reverse image search that helps identify the original source of an image