Hmmm...I'll start with the ones I actively post in.
-Geekhack (I'm a bit of a mechanical keyboard and other input device enthusiast, and I have yet to find a better forum for that sort of thing.)
-GameSpot (I'm just there for a union board and the PC and A/V discussion boards these days to give people advice, though I sometimes dive into legacy gaming.)
-SimHQ (Ever since Frugal's went down, this is the place to go for simulations of all sorts, but especially combat flight simulations and gear to play them with.)
-Good Old Games (There's plenty of good classic gaming discussion to be had there, especially on the games that they sell.)
-Through The Looking Glass (It's another gaming forum of choice for me. More focused on specific PC cult classics, though.)
-Underground Gamer (What a surprise-it's another classic gaming forum, though it's part of a tracker.)
-The Board (It's one of those secular, small forums you'd never know about unless someone told you about it. Random stuff gets discussed there, but I'm just there for the gaming talk. Notice a pattern?)
Then there's countless other forums that I just lurk for information, of which this place was one until a few months ago.
Other than information, I've found the buy/sell/trade sections on some of these boards specifically to be quite useful. I've acquired so much gear I wouldn't be able to find or afford otherwise, ranging from near-top-of-the-line flight simulator equipment to my first Stax setup.