I've seen quite a few Flickr API mash-ups, but this has got to be one of the most creative. Idée Inc. has created the "Multicolr Search Lab" that uses a collection of 10 million Creative Commons licensed images on Flickr to match photographs to a user-selected color palette.
As the user selects up to ten colors from the palette on the right, the mash-up searches an index of matched colors and pulls up results in the form of thumbnails, each linked to the image's Flickr page. Not only is this just plain fun, but because of Creative Commons licensing, an artist can quickly find images to use in a project (as long as he/she gives credit and shares alike). Idée's Piximilar, the engine behind this app, seems to be a very powerful and groundbreaking image analysis system. It will be interesting to see how this algorithm is incorporated into future projects. For now, you can check out Multicolr at http://labs.ideeinc.com/multicolr/.