This forum: is actually fairly well known among the others for the amount of drama in the games. Of all of the forums I know that do Dom3 games I think the games here take the longest to setup, and the longest to play. The Chat (webbased IRC) here never quite seems to catch on but its pretty good for a meeting place to setup a blitz game without interference. (its that clickable link at the top of this page that says "Chat")
Invision or the Dom3Mods forum was started by people who were banned from here. Their forum and their #dominions IRC channel is well filled with veterans of the game who can discuss strategies and tactics of the game
SA the SomethingAwful forum: refer to themselves as goons. The have their own IRC channel and their own tracker server which is excellent. In general its more "play for fun" than "play to win". There is less response there if someone drops from a game or goes AI. Lately their games have also shown more variation than most in trying new things. Yes to play with them you just join the SA forum for $10
Wargamer: as the name suggests these are the "grognards" or wargame crowd. Dom3 is considered "not really a wargame in our usual style but still lots of fun to play". Almost everyone is a newbie to Dom3 but experienced in wargames. An interesting mix
GalCiv2: the StarDock forums (there are many of them) do have their own share of Dom3ers. Its only natural since Dom3 often comes up in comparisons which makes people curious. Their company and this company have always gotten along well. The games there tend to be private with a few example games. Many of the forums have these and they are usually called Lets Play, as in Lets Play Dom3. which is alot like our AARs. Lots of discussion about what is going on the game, usually in a story format, and lots of screen shots. They also have their own chat
Qt3 or QuatertoThree forum: these are more developer types and computer career people. They actually seem to have more fun RPGing the game than many do
RPGcodex: programmers and coders. They play Dom3 games but the discussions tend more toward HOW the game does things.
TheGaming Den: hard to define. small group, a few games, fairly standard. Lots of solo players.
Bay12Forums: same as above
There are others but many of them would not be thrilled at being mentioned here. They all know we exist.