The best site ever for downloading free mp3s of great music is http://www.buffygalaxy.com/audio.htm.
It’s an Italian website but you don’t need to speak or read Italian to navigate it; it’s pretty straight forward. It has pretty much every song ever played on Buffy the Vampire Slayer. They used so many great songs on Buffy by totally obscure artists – I could never afford to buy all their albums just for single songs. This site has all the songs by episode – awesome huh?