The three top paid forum softwares are Invision Power Board, vBulletin and XenForo. I think XenForo has definitely beaten both, because it has started using jQuery at the earliest stage. vBulletin is one of the most customizable board softwares though and offers a large opportunity for developers. Invision Power Board may have a nice and clean look, but is extremely expensive and must be renewed every 6 month! Which do you choose?