I umpteenth the phpBB praise.
I've been on three or four phpBB boards--no problems that I can think of for each, and they can shine up nice and purdy.
(Stock, it looks like this:
preston.sc.edu. With some work, I've seen them get about as slick as this:
Pokémon ZEO!. Yes, I still go to a Pokemon site. Don't hate the playa, hate the game.
)