To get this started, I’ll paste some past comments from this thread:
liberty student:vBulletin took a huge step back about a year and a half ago when they were bought by a publicy traded firm (Internet Brands), and the latest version of the software is not even close to their legendary quality. They have a lot of customer satisfaction issues from what I have observed.
The previous lead developer of vBulletin and others have quit and created their own new forum (written anew from the ground up).
(see also: http://xenforo.com/community/threads/a-statement-regarding-the-current-litigation.7567/ )
I think it looks great. (adoption: http://xenforo.com/community/threads/show-us-your-xf.9236/ )
Otherwise, there are a number of good commercial ones too. ex. http://www.invisionpower.com/
And if Mises wants to have open source free ones, there is phpbb and mybb.
I’ve deployed and I’m administering a xenforo forum. It’s not worth paying for. Free stuff works fine.
I am currently migrating a free forum (phpbb) it is a joke. Paid forums are much better, the quality standard is night and day.
I would say MyBB is better than ‘night’.
Conclusion
MyBB is a really good alternative to all well known forum softwares, and the community around it is really dynamic. It combines an easy to use administration interface with a simple and shiny appearance. Without attempting to support the entire range of features we could classically find in proprietary software, MyBB is a perfect forum for medium and large community. A sufficient security model is well implemented and ensure your forum not being invaded by hordes of spammers.
With more than 400 plugins and modifications, you will have a great choice to customize your MyBB installation and fit your users’ needs. It is also well known for ensuring a high quality migration of existing forum, as the MyBB developers team provides the MyBB Merge System, that helps you replacing your old PhpBB 2 or vBulletin 3.8 installation with a new and modern bulletin board system. It could also be perfectly used in case of merging two existing boards, that is quite rare today.
And to conclude with another good news, it is open source and is proud to be! Congratulations to the MyBB team, that has made a really good job.
http://www.forum-software.org/mybb/review/conclusion-advantages-opinion
The Pirate Bay forum runs on it, with 122K registered users. http://forum.suprbay.org/