Aside from "General Discussion" my favorite forum here at EN World is the "Books/TV/Movies" forum. It's a great place to find out about books & movies before you shell out the bucks to buy 'em!
Until recently, it was definitely the "Books, movies & TV" forum. But now that I'm starting to DM, House Rules has been my favorite. Unending stream of inspiration.
Crikey, do I have to pick just one? I guess probably the Story Hour, for not only tons of entertainment, but also lots of inspiration for my own game...