In that case the best free forum software would be PhpBB3 located at:
create forum
It is a lot like vBulletin and I do mean a lot like it! At one time it was a reverse engineered clone of vBulletin and they were ordered to take out any and all vBulletin coding. Yet, it still remains one of the more impressive free forum softwares available - in my opinion.
It's like the poor man's vBulletin, when you want the best but can't afford it - then get the next best thing (they even have a vBulletin skin available.)