Invision Power Board: Forum Software Free to Use: I’ve used a lot of BBS software, but I’ve never seen one this good before. I first ran into this as the backend behind the Movable Type Support forums. It installs easily (one page of options to get it running), and the number of administrative features is utterly staggering. They let you do things I didn’t even know I needed to do.
The software is free if you display the Invision copyright, and $59 a year otherwise. Invision also makes the Invision Power File Manager. Haven’t tried it, but the screenshots look dandy.