phpBB is an open source forum system. phpBB is the most-used free forum script on the Internet.
From the Author
phpBB is a high powered, fully scalable, and highly customisable open-source bulletin board package.
phpBB has a user-friendly interface, simple and straightforward administration panel, helpful FAQ, and is the ideal free community solution for all web sites.
Authors website: http://www.phpbb.com