Hasn't anyone considered
http://www.phpbb.com/?
It's free.
I've never worked in forum code before, but this is the way I'd go, if I were starting out with something new.
Simple Machines is interesting; never heard of it. But it works well, if on a few different assumptions.
Goes to show you how amazing the php/mysql thing is. There'll probably be a whole other world of this stuff in five years.