Two more online music-buying services (in addition to MP3Tunes) offer digital music without limitations on personal use:
Audio Lunch Box
Tracks are 99 cents, full-length albums 10 bucks. No membership or subscriptions. (Thanks Kevin!)Emusic
Monthly subscription, flat fee for a certain number of song downloads (10 bucks for 40 songs a month comes out to 25 cents cheap per song.) 50 free songs on signup. Best value for frequent buyers. (Thanks badger!)