Last.fm is the go-to-site for anything music related. I'm trying to think of the right word to describe it but really, there is no one word or phrase that best describes it. The official slogan "The Social Music Revolution" touches on the essence of what this is all about.
I first came across this site when a friend of mine suggested I check it out; knowing I am the type of person who loves to organize everything nice and neat. I was first taken by the ability to track your what you listen to on iTunes. You create a profile on the site and the more music it tracks, the more it gets to know your music taste.
Last.fm recommends artists weekly, based on what you are listening to. Of course there are a whole host of groups you can join, catering to different interests, and these groups are an incredible network for finding virgin music. You can also search for music by artist, album, song, you name it.
Each artist has a page that displays other similar artists as well as an overview as well as listener stats, charts, album info, and more. You can listen to clips from songs, and sometimes there are full tracks to listen to and/or download. It's wonderful. I could go on and on but really you need to just check it out.