The much-hyped music service Spotify recently finally launched in the United States after being Europe-only for a long time. I've been checking it out, and I absolutely love it. I signed up for the paid account, so I can't speak for the free offering, but in my short time with it it blows stuff like Napster, Rhapsody and Slacker Premium, all of which I've used, out of the water.
It automatically imported all my iTunes music and playlists, and it seems to keep that up to date, which is huge for me, since it removes a lot of the effort in integrating it into my music listening. The paid version also allows me to listen on my phone, and even cache music onto it for offline listening. The selection, from what I hear, is the best of any similar service, and I haven't run into any limitations with it.