For anyone interested in podcasting, I highly suggest that you check out Podsafe Audio. Podsafe Audio is interesting in that it is a collection of music that is shared under a Creative Commons license. In other words, it is music that can legally be used in mashups of all types, but with a focus on podcasts. Even if you're not interested in podcasting or copyright, Podsafe Audio is a good place to find socially aware and progressive music by smaller artists (usually unsigned, but some signed). Organized by genre, it's a great way to experience a wide variety of music.