Having spent way too much time in HiFi speaker forums, here's my breakdown:
AVS - good for home theater and electronics discussions - very active forum (possibly the single most active audio related forum) with some good info, but the speaker/subwoofer sections degraded into too much fighting, BS and drama for my tastes quite a while ago - I go there if I want to find info on TV's, receivers, DVD players, etc
AudioAsylum - this is where a lot of the far fringe of audiophiles hang out, so if you're into exotic cables, funky tweaks and expensive stuff, it's a good place - I don't find it too useful
AudioGon - this is mostly an online classifieds site for audiophiles, but they have a forum as well with some decent discussion, but I've never gotten into their forum
AudioCircle - I like this site, but it is biased towards the local companies - they have forums for a number of smaller HiFi companies and many of them are quite good. There are also some decent DIY sub-areas on the forum. My current favorite general HiFi site because of the good collection of folks.
Audioholics - a home theater review site that also has a forum. I haven't bothered much with their forum as a lot of the posting seems to be back-and-forth about their reviews.
Home theater forum - a decent site for basic home theater stuff, but the speaker section isn't that exciting and I gave up on it after a while because it seemed to largely be newbies asking the same newbie questions
Home theater spot - another home theater forum similar to the one above, but with sub-areas dedicated to the discussion of particular brands (imagine if there were Grado/Senn/etc sections here)
Home theater shack - yet another home theater forum - I haven't checked it out other than the Room EQ Wizard section - looks like AVS, but low traffic
Sound&Vision - home theater forum - I haven't really checked it out, so I can't say much
DIYaudio - as the name implies, a forum for all things DIY for audio (a bit like HeadWize) - lots of technical discussions on building speakers, amps, etc
Madisound forum - another DIY forum, this one focused on speakers - it's good if you need some quick DIY speaker advice
Company specific (all are naturally biased towards their products):
AV123 - probably the most active of the company specific forums - you'll see me here quite a bit
Ascend Acoustics - I'm here quite a bit too
Axiom
Hsu research - I gave up on this one
Aperion
...I'm sure there are more