are two of the best free forums out there..
But I think invision is the way to go... it allows you to link forums to other url's...
check out http://iscream.homelinux.org
and click on the linux stuff... this is just a sample forum that I am working on right now... thought I would show you it can be done..
the only catch is if you do not want to purchase a license, you can't remove the copyright stuff they put in the script.. not a big deal though..