Mostly here, but lately I've been spending some time at Alderac's Spycraft boards too. Occasionally RPG.net, Wizards, or any of the various D20 publishers.
Well, I occasionally check out RPG.net, but that is becoming increasingly infrequent.
While I agree that the WotC boards have a very different mind-set than EnWorld, I've been pleasantly surprised at some of the cool links and ideas I find over there sometime. The kids are alright.
That being said, it's very rare that I post there. Most of the questions posted are rules-oriented and quickly answered by admins and/or "experts".
I very occasionally visit Monte Cooks boards and the Mongoose Boards (can't remmeber what they are called, I guess that tells you how much I visit).
I'd LOVE to find a message board dealing specifically with Dying Earth, but no luck so far, and I have not yet been ambitious enough to start my own. Why start such a message board when the sun could go out tomorrow?