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Here, obviously.
Wizards of the Coast
RPG.net (not so much in the last year, though, and even then sparsely)
Green Ronin (mostly on the M&M boards, but I mostly lurk there)
Boardgamegeek.com
HeroscapeHQ
KCRoyals.com (yeah, I'm a glutton for punishment)
Just started posting on Nothingland
This is where I do the overwhelming majority of my posting.
I lurk at rpg.net and The Forge, and post at Dragonsfoot and several DF-spinoff sites for people who think DF isn't hardcore-oldschool enough (yes there really are such people, about two dozen of us...).
In order of number of posts (approx):
AV Forums (home theatre)
here
Nothingland
Civic Type-R owners (car)
Monte Cook's forum
Maxminis (not so much recently)
AEG's spycraft forum
rpg.net