Well, there's always Last.fm, which gives you similar artists as chosen by other users, and then their similar artists, and so on. You get the picture. But it is quite a useful little site, and it has helped me find half my Japanese music, most of which I love. It's only ever about music, so none of that whinging and camwhoring you get from social networking sites. You can listen to previews of some of the artist's/band's songs, see what kind of music they play, find out how popular a band is, when and where their next concerts or gigs are, their most popular songs etc.
So yeah, quite a good place to start. There is already a thread about Last.fm but I can't be bothered linking to it right now, it's mostly about what I've said here.
And here's the link for you lazy people like me:
http://www.last.fm