For forums, the big three names are vBulletin, Invision Power Board, and phpBB. This forum uses vBulletin because it's the best; it offers superior extensibility and ease of administration. The code makes sense, too. IPB at one time was available for free, but I'm not really sure anymore; IPB has most of the features of vBulletin. phpBB is a free forum software. You get what you pay for, though over the years phpBB has gotten much better than it used to be. I'm inclined to believe that phpBB's template customization system is still as aweful now was it was when I last worked with it since I haven't seen any phpBB forums with any sort of changed structure.