As Scott Dockendorf mentions, we launched the beta version of a new online music community site today called Crossfader in partnership with »telligentsystems, the guys responsible for CommunityServer. From the site: "Crossfader is an online community for the global electronic artist. We are building this tool to help you connect with people who share your love for creation, great music, technology and modern culture." So, if you're into hip-hop, techno, or trance, this sounds like a site that should be added to your short list.
On a somewhat related note, I'd be remiss if I didn't mention Rockstar Games (of Grand Theft Auto fame) recent Beaterator, a real-time music sequencer that runs in your web browser. This thing is addictive. I know the guy who was intimately involved in putting the site together, and I'd say he's done a great job.