After a bit of an absence I'm getting back into the whole Last.fm thing again. It's a sweet little website where you download a plugin for use with your favourite media player and then it compiles a chart of your favourites.
You get your own personal page, mine's here and it's lists your musical "neighbours" according to how close their tastes are to yours and these change as your database of music grows.
The neat thing is the playlists/radio. You can "tune into" your own personal radio station, your most loved tracks and even other people's. If you like someone else's song, you can mark it as a favourite and then it appears in your favourites radio station to listen to anytime - even if it doesn't exist on your PC anywhere - neat huh?
The only downsides are occasionally the servers get hammered and you can't listen to radio, also it only logs mp3s as far as I can tell as that's the only format that has tags that the website can read.