There are only two reliable options (for me) AFAICT:

1. WebGUI
2. MovableType (yes this is a CMS)

WebGUI is limited to Apache unfortunately and I had to eliminate it since the site I'm planning to convert is on IIS/Windows Server 2008 (no, I will not change the host or install Apache on that). It's a pity, while the only needed thing is a litle abstraction layer.

For MT, there are two options: MTOS & MTPro. MTPro has the community features which MTOS lack but MTPro is free for power blogger type which fits for most I guess and it has a forum feature (which is the good ole blog under the hood)

• MTOS: http://www.movabletype.org/
• MTPro: http://www.movabletype.com/

However, if the only thing you need is a forum software just stick with SMF or phpbb.