Glad to see a few people have brought up Jamendo already, but I'm really surprised that no one has mentioned ccMixter [0]. This is one of the really early sites to try and address this exact need. They have an enormous back catalog of open-licensed music designed specifically for mixing and reuse, and a still quite active community of dedicated users (though the site design needs a bit of a refresh).
[0] http://ccmixter.org/