If you're just looking for chart and pop music, try:
http://www.noisematrix.com/
For the more dance-oriented amongst you, go for:
http://www.homeofmusic.com
...but be prepared to be *incredibly* annoyed by the site design (right-clicks and keypresses disabled!)... if you can put up with it, all the latest "choons" are there..!
For the slightly more brave, IRC is the weapon of choice. Go grab a copy of mIRC from any popular download site, or straight from the horses mouth at:
http://www.mirc.com
There are plenty of MP3 and DivX (movie) sources on IRC, but each of the chat rooms has its' own idiosyncrasies, so I'd advise you to lurk for a while to become familiar with the system.