You might say that I'm very biased with my statement but that's what I have experienced. I considered vBulletin as the most powerful forum software I have encountered. Since I started my job last year, most of the time was spent on on vBulletin and sometimes I do on coding website but it just XHTML, CSS, and JavaScript. On vBulletin, I do from installtion, skinning and sometimes hacking of it to add new functionality and features. Almost of the big sites (SitePoint, DigitalPoint, Web Hosting Talk, Admin Zone, Dani Web... so many to lists) used it. From vBulletin site: vBulletin is a powerful community solution that makes it easy to add a forum messaging system to your web site. vBulletin is developed using PHP and MySQL (a highly efficient open-source database). This solid back end technology allowed us to develop a product that is both amazingly fast, yet reassuringly stable. You can view its features here. I have never seen another forum software that has many powerful features as vBulletin. One of the thing I like most on it is it's templating system, plugin system and its scalability. And, others already used it as a Content Management System - that's how scalable vBulletin is. When it comes to the free one, I prefer SMF then phpBB. We cannot argue that phpBB is the popular one but for me it's SMF that has a greater feature.