My main "base of operations" is
www.TheJawa.com (I'm the customs moderator there) but we only have a few regular posters.
I definitely think that JD.com has the best bunch of customizers out there, I love the real community vibe here and, aside from the high level of talent, there are also great comments on everybody's customs.
I also have accounts at
www.FFurg.com (haven't posted there in a while, I probably should - they're a great customizing community as well);
www.GalacticHunter.com (I usually update my customs page here, but most of the customizers at GH.com post here too, so I usually comment here instead);
www.CommandoCustoms.com (I just post my Microman customs here);
www.RebelScum.com (I post random things but mostly use the forum for the Trades Section, if at all);
www.SandTroopers.com (hardly post here anymore); and of course,
www.thejawa.net/NJSWCC (New Jersey Star Wars Collector's Club - it's pretty much just me and Mitch (aka Tykrazen))
I find it hard to keep up with so many different forums. Nowadays I focus on TheJawa.com; NJSWCC; and JediDefender.com. I like the other sites and they have great people, but it takes way too much time to keep up with everything.