http://fallout.bethsoft.com/
The forums aren't much to look at, though. It's attracted the most cynical, the worst of the worst and the first few days saw little in the way of constructive discussion. Soon enough the Bethesda fanboys began a large scale counteroffensive, leading to a very unpleasant and depressing atmosphere.
Things are getting a bit better now, though.
Fallout 3: A post-apocalyptic RPG set in the Bethesda forums :P