I don't know what "extreme metal" is nor have I found a Metal netlabel yet but the site with the worst redesign strategy in the free music world has some great ones. Might not be the style(s) you are into.

http://www.jamendo.com/list/a41418/whispers http://www.jamendo.com/list/a37727/the-awakening http://www.jamendo.com/list/a27926/darkshine http://www.jamendo.com/list/a42180/coma-circle-lp http://www.jamendo.com/list/a76571/angelic-asylum http://www.jamendo.com/list/a43370/anno-domini-mmix

Thanks, I'll check them out and play around with jamendo. Here's a nice site for distinguishing subgenres of music: http://www.furia.com/misc/genremaps/engenremap.html