OWL multimedia has launched an audio similarity search engine stocked with 10,444 CC-licensed tracks from ccMixter and Magnatune, with many more to come from other CC supporting sound repositories.
You can search OWL via search.creativecommons.org but its real power is finding new music through music. The OWL website allows you to choose an mp3 on your hard disk, which will be used to show you tracks that sound similar to the mp3 you provided. You can select a segment of a track to search on and of course you can limit your search to tracks with licenses that permit uses you require, e.g., commercial or derivative use.
This should be an amazing resource for music discovery in general and remixers in particular — try it out!