I picked phpBB as forum software for the forum on Darkfall Crafters. There are plenty of forum software these days, so the choice is by no means automatic. To me phpBB however represents a solid piece of free (as in speech) software that is built for customization and has many user-created extensions. The only thing I miss is support for threaded discussions.
Overall the forum was fairly easy to integrate with the rest of the site (although that site, and this one, are long overdue for a graphical overhaul). There are several modifications left to do (better integration, SEO, adding handy utilities/tools), but I'm saving those for later since there is no point in spending time on them now if it turns out that the demand for the forum is low.