Sometime back Koffee Brown started a thread about ripping audio files to CD for use in her fitness classes.
I thought of that thread when I stumbled across Epitonic.com. This site has an extensive collection of categorized music placed online by musicians for the express purpose of being freely downloaded. These are mostly artists from smaller and independent labels as far as I can tell.
Much of the Dance, House, Pop, and "Drum and Bass" categories would be useful background music for a fitness class.
There is a alot of very unusual stuff there from all sorts of genres. I have spent the last few minutes listening to bands named "Death Cab for Cutie" and "I am Kloot".