You have IMHO two choices Xforum or PNphpBB2. Xforum is what is being used on this website and PNphpBB2 is being used at postnuke. I used to use PNphpBB2, but I found it to be a resource hog and at times very slow thats why I am using Xforum now. PNphpBB2 has heeps more features though. To find them do a search on google for either "PNphpBB2 postnuke module" or "Xforum Postnuke Module".
Hope this helps.
I hate to break up the fun with a stupid support type question. The real place for me to post this would be on the PostNuke web site, but I can't figure that site out. I've just installed version 0.7.2.3 of PostNuke, everything is running fine (I'm using WV's HarmonyX theme), but I can't find the module for the forum, and I've read that a forum is supposed to be available. I really would like to find out how to access it (or how to download and install it). I've been using Invision's board, but when I make the switch to PostNuke, I'd like to have something more integrated (and so they wouldn't have to log in twice).
I know that this is probably the wrong place to be asking this, but if anyone can help, I sure would appreciate it.