Online music services have sprouted up like so much ragweed in an unplowed field in early spring. It's getting harder to keep track of them--Apple's iTunes, Roxio's Napster, RealNetworks' Rhapsody, Virgin Digital, Yahoo!'s Launch, and the list goes on and on. There are also online music stores from Wal-Mart, Best Buy, and other retailers. Some providers market premium subscription services, others sell single digital downloads, many offer both.