Wellll I don't really browse a tonload of forums in particular, but I am a prominent member (by that I mean I go there every day, not that I'm hot stuff there or anything...) on Anime Planet (cuz anime's great) and Yu-Gi-Oh! Cardmaker (I only made cards for the first year, then I just stuck around for the community). I'd recommend joining both if you like those things, unless you don't, in which case I'd say pass the hot potato elsewhere.
What about yourself, any cool places I should know about?