Heres a site that i thought id share with you. Its called pandora.com, you enter your favourite artists and it finds music thats similar and plays it, ive found some good bands on there.
There's a site that I use called music map that shows you bands that you'd probably like based on how other people like bands. So if you entered Slipknot, it would give you bands that people who also like Slipknot like. http://www.music-map.com/
yep. Also you could use last.fm and see whats on the "similar artists" page.
Though I believe last.fm does it based on what people are listening, so it might not be exactly 100% accurate, but it works pretty well.