Spotify. They've finally managed to bag a deal with one of the US major labels, for the US market. Its only a matter of time before the others wake up, and the service launches here in the States.
If you don't happen to know anything about them, they're based out of Sweden and, besides the still-to-be-heard-from Google music service, are tops on the list to challenge iTunes' domination of the digital music market. I've been lucky enough to have used the service, and I absolutely love it. For more info, click here.