Haven't tried it myself, but it seems to be well-liked. (
http://www.last.fm/)
According to this VC, last.fm is better...
http://www.techcrunch.com/2006/01/23/655/
But if you don't like one or the other you can try a mashup of both...
http://www.techcrunch.com/2006/03/29...gethersort-of/