Emusic seems to have a pretty good selection of songs, all available in MP3 format, so they can be played anywhere. It appears you have to sign up to see their whole collection (I haven't yet, but plan to when I have time to browse), but they apparently just posted a large collection of Sun Records early tunes, and they have Tom Waits new album, some CCR, Patsy Cline, Willie Nelson, and apparently Frank Zappa is well-represented. The free-trial seems to genuinely have no strings attached, so I'll soon be giving it a try.