I'd say 99% of forums on the internet use some type of forum software. Like vBulletin, phpBB,
Invision Power Boards, etc. This forum uses vBulletin and its probably the most popular forum software available. vBulletin just got bought out by a different company and they are going to be releasing a totally re-designed v4.0 soon. I like IPB's front-end best, their designs are really clean and easy ...