A few posts ago I raised the question of how to introduce yourself to things on the internet that you might like but don't yet know that you'll like (or something like that). Musically at least, Twisten.fm might be worth a look. Here's what it does: scans through Twitter for tweets about music, sticks them up on its front page, and - the crucial part - gives them play buttons. Brilliantly simple; like all the best ideas.
Downsides? Just a few: the occasional glitch in playback, not being able to leave it playing tweet after tweet like a radio station, and possible exposure to bad prog metal ('Knights of Arabia by Kamelot' should have been a clue in hindsight, mind you), but nothing fatal, and their Get Satisfaction profile says they're working on continuous play.
As for where the music comes from, Grooveshark, the other side of the mashup, looks well worth further investigation in its own right - it's a music-based social network, with a Pandora-ish player, and the option of uploading your own music collection to the cloud for listening on any computer; the last of these could prove very handy (not to mention being a potential USP/POD for the service).
More online listening ideas at lifehacker (and here's the post where I happened upon Twisten).
UPDATE: (27/2/09) Twisten have already sorted the continuous play thing. Excellent stuff.