The forum or website I've been visiting most is the Civilization Fanatics Forums. I've been a member there since 2003, and like here read there daily even if I don't post a lot.
Of the two boards I post on regularly, that's the other one for me. I also registered back in 2003. It's kind of the opposite of here in that I'm still there to discuss the game and the amount of time I'm there goes up and down depending on whether there's news on a new game (I do post in the World History forum a bit. I used to be in the "other games" section for Mafia games, but my interest kind of died out with that).
Hell, way back in the day, I was on a shit ton of forums. I started off at a forum to discuss the computer game Starfleet Command. From there, I found here - mostly to discuss Enterprise at first, but I stayed for the off-topic sections. I know I registered in the second season right after the Romulan episode, but I tried to log in a year later for the Xindi arc and couldn't get it to work, so I re-registered. I also was on startrek.com's message board, but I think I realized always that it was terrible. I think I posted a handful of times at EAS's forum under a different name.
On the Civilization side, I was on a site called "Creative Design Group," which was devoted more to modding, but also had a nice core group of people who were fun to hang out with (it was also very international, even compared to here). I was on CFC and Apolyton and would occasionally post on a few others. Over the years, the number of boards I would post on contracted as the company that made SFC went out of business and that closed and the people at CDG just sort of disappeared. Now I'm just here almost always and CFC usually once a day. The sad thing is having fewer sites to go on hasn't decreased my internet usage, which seems more free time/boredom related than anything else.