Shelley Powers posted a a brief review of eMusic, which differs from other music downloading services in that the tracks are available in the MP3 format and are unencumbered by DRM of any kind. They offer over 600,000 tracks, although major label current releases seem to be missing (not that there’s anything wrong with that). They offer a two week free trial during which you can download up to 50 tracks, so it sure seems like it’s worth a shot.