Each year, I try to make a gift for the world. Last year it was the Infinite Canvas.
This year, it's the Clicks Image Search.
It's more of a journey than a search. Click the save/thumbs up button on images you like (or are like what you're looking for), and drop/thumbs down the ones that aren't what you want. If you don't see images you want, start with a search.
It develops a model of what you're looking for, and tries to give you more pictures like that.
It's based on tags and a naive Bayes classifier. If you want to see and adjust the model, there's a spot at the bottom. It seems a bit buggy in IE, but it works fine in Firefox.
Try it out, and tell me what you think! http://www.existencia.org/clicks/