seems pretty interesting. You can upload a sound file and their servers will analyze it and spit back an XML file.
"The Echo Nest’s proprietary music analyzer API automatically analyzes audio (e.g. aif, wav, mp3, m4a), and generates an XML file that describes the musical and structural content of the music. That representation can help power music visualization, music games, artistic installations, content-based and DJ applications with a much deeper level of music understanding. "