indieish.com -- cc:365 -- track of the day with a Creative Commons license, usually accompanied with a link to the netlabel release.
GarageBand -- Face Off, actually two free tracks a day, plus the added twist that you get to vote (registration required) for your favorite. One voter wins around $200 worth of gear every day. The favorite band of the day can go on to win a recording contract. (Warning! highly annoying intro on Face Off page -- turn your volume down!)
Napster -- track of the day (registration required, unless you solve the puzzle). They have the last 7 days available for download.
emusic -- just today's track. I find that emusic has the selection I enjoy most.