A while ago I wanted to set up a forum for a few users and after a few days of research chose myBB.
It seemed to be the best free choice available then, with good theme and plugin support, and simpler than phpBB, SMF, etc.
Note that this was before the more dynamic forums started appearing.
I know they are working on 2.0, which looks much more Discoursey but it's currently in Alpha.
In the end I went for a Wordpress forum for easier intergration, but I do remember myBB seemed a nice forum at the time.
'd by a few days
D'oh! Must be my Olympic-rated reflexes