Last.fm also has web radio stations of a bunch of different genres at 128kbps (CD-quality) that stream through an opensource, extremely small player. Last.fm supports tagging, blogging, etc.
It also generates listening statistics for you, following trends automatically.
To me, however, it's the community surrounding Last.fm that seems most wonderful. I never jumped on the MySpace bandwagon--I'd rather stick with real friends, not virtual avatars. But Last.fm is your "music friends." I like that: it's a way to find out about bands that I'd never find otherwise.
Which is the other plus of Last.fm. There's a massive library of full-streaming and 30-second sample streaming indie songs that are really good, but don't get publicity because they aren't on a major label.
Finally, here's my Last.fm profile. If you like what I listen to, add me to your friends list.
Last.fm also generates a list of the most recent songs you listened to. Here's mine as of this posting:
Edit: Actually, the above list isn't as of the posting; it changes automatically as I listen to more music. What you're looking at now is the most recent songs I've played on my computer.