I am always on the quest for music to listen to when I am working out.
JogTunes is a great site that, based on your own workout pace, provides the bpm (beats per minute) of a ton of songs. This enables you to create the perfect workout mix. Don't know your pace? Use their simple Pace Tracks work sheet to figure it out. Once you know your pace, search through JogTunes to find the songs with the bpm that suits your workout. JogTunes is free to search, but you are directed to iTunes to actually purchase the songs.