LastFM seems very interesting.
It’s a kind of social music site where users can share what they’re listening to (not the music itself, just the info) and in that way interesting info is gathered. E.g. The popularity of music is monitored, and you can find similar artists to artists you like. I’m gonna keep an eye on it, and come back some time with an update on how good it is.
Here’s a link to the LastFM user charts: http://www.last.fm/charts/