Simple Machines Forum
I've been running a small HiFi/HT forum for about a year and a half - AVForums - and recently got rather fed up with phpBB's bugs. Just before I spent the bucks on vBulleting, I discovered SMF and thought I'd give it a try. What a nice surprise - clean and logical management interface, easy to install "packages", and a much more modern UI (very similar to vB in fact). Also it appears to be a lot more secure than phpBB - something you really should worry about given the rise in comment spam lately.
In the process of converting my phpBB database (probably due to the fact that I was using phpBBFM and not a vanilla phpBB installation), I encountered some strange errors for which I could find no answer readily. Those included some topic notifications and group permissions which failed to migrate over to the new format. If you have these issues, do not ignore them and just carry on (even though the bulk of the data might seem to have copied). Rather skip that particular section only by editing the phpbb2_to_smf.sql and deleting only the sections that I did not want to import. I went over all the settings (moderators/permissions in particular) in the SMF Admin console afterwards - a lot of the migrated settings are actually redundant and could be deleted. So far I am very impressed and can highly recommend this package to anyone who is running a phpBB instance or thinking of starting their own bulletin board and can't justify vBulletin.