Daytrotter invites (mostly) indie rock bands to play a session of four songs which can be downloaded for free from the site afterwards. The concept is simple but works very well. "They are all live, no overdubs, straight to tape. What you hear is what happened in the room that day." The site's around since march 2006 and they've collected an impressive archive of 1500+ songs which grows at a rate of 20 a week.
How did I manage to miss this for so long? Listen to Maritime, they're good.