WordPress forum plugins suck. They are made for 5 year olds who've never used a real forum before. Your best shot for free forum is:
It's an opensource software and very powerful as well.
Unfortunately there is no plugin to easily integrate that into WordPress. OP was inquiring about a WordPress plugin. Yes, you could use a plugin to bridge them, but you'd still have to customize a forum like that to match your WordPress site. Not to mention the security implications of using a bridge. If the OP wants to use a forum that's 'outside' of their WordPress install, then more power to them. But that's not the impression that I got. And without proper visual and function integration, it really looks sloppy. People that use applications like SMF and WordPress together without integrating them properly have very poor design skills. Not to mention having to register and login to both sites, without using a bridge. I just cringe at the terrible design whenever I come across one of those sites. But I guess to each their own.