iTunes Store - Apple
Build a collection of digital music, movies, TV shows, iPod games, audiobooks, and podcasts from your computer.
http://www.apple.com/itunes/store/

RealPlayer Music Store
Browse the latest artists and choose from millions of songs. Legal music downloads.
http://www.real.com/musicstore/

CD Universe
Shop online for music CDs, movies, games, and novelties.
http://www.cduniverse.com/