I've been using Audioscrobbler (last.fm) for a few months now, and find it fairly useful. Basically, you install a plugin in your music player and it reports which songs you play. Based on this data, the site can then recommend other music you might like, based on what other people have played that have a similar playlist. You can see this data on the site, or you can play the last.fm radio, which is customized based on your stats (and you can filter out music you don't like, of course). Its a handy site to learn about other music you might like.
They have previews of popular songs by most artists to boot.
Spiel from the site:
You get your own online music profile that you can fill up with the music you like. This information is used to create a personal radio station and to find users who are similar to you. Last.fm can even play you new artists and songs you might like. It's addictive, it's growing, it's free, it's music.
=0=
We have created a little group for SE players to join:
http://www.last.fm/group/Space%2BEmpires