FUDforum

FUDforum is a highly customizable Open Source web forum released under the GPL license written in PHP. It has a large feature set while maintaining the ability to generate forum web pages extremely fast. The easy to use administration control panel allows the administrator to easily configure and control the many features available. The forum also includes an interactive help package, which helps to familiarize users with the full potential of FUDforum.

Homepage: http://fudforum.org/