Korn, KornBSC [their now-defunct fanclub, the new one is coming soon,] echoingthesound.org [the "unofficial official" Nine Inch Nails forum,] AFI, and now, Evanescence [pending.]
I'm on two Paramore forums but this is the only forum I am active on. I only use the Paramore forums to read up on news. I only have like 20 posts in 6 months.
TWOP.com (Television Without Pity), Woxy.com (Online music station + more current events/media discussion), and imdb.com (Reading up on random movie/actor tidbits)
I think I am most active on here but every few weeks I check up on Woxy. I sometimes read up on what is said at TWOP, but that is more every few months than anything. I only go to IMDB when I am looking up a specific actor/movie and that is when I read the forums, but that almost never happens.
I might be forgetting something but that probably means I visit that forum even less than IMDB, so it might not even be worth mentioning.
Actually, I think I've made a mistake with signing up on that Evanescence forum. What a terrible place, your posts have to be approved before showing up and I have some from ****ing noon which are still not approved. No wonder Amy Lee is a bitter bitch, her own messageboard probably pisses her off.