Prior to vbulletin, feed the geeks used phpbb. Phpbb is a free forum (you have to buy vbulletin) that is very good. It has a plethora of mods available for it. Another alternative is Invision. I believe they offer a free version as well as one you have to pay for. You can try it out at
InvisionFree (Free message board hosting).
vbulletin, phpbb, and invison are the big 3 in my opinion. If you are going for a free forum, I would recommend phpbb. If you are willing to pay, I would suggest vbulletin.