phpBB
phpBB is a high powered, fully scalable, and highly customizable Open Source bulletin board package. phpBB has a user-friendly interface, simple and straightforward administration panel, and helpful FAQ. Based on the powerful PHP server language and your choice of MySQL, MS-SQL, PostgreSQL or Access/ODBC database servers, phpBB is the ideal free community solution for all web sites.
Homepage: http://www.phpbb.com/
Documentation: http://www.phpbb.com/support/
Support Forum: http://www.phpbb.com/phpBB/
SM Forum
SMF is all of the above. SMF is a next-generation community software package and is jam-packed with features, while at the same time having a minimal impact on resources.
Homepage: http://www.simplemachines.org/
Support Forum: http://www.simplemachines.org/community/index.php
XMB Forum
XMB is a fully scalable, highly customizable Open Source message board software. XMB has a user-friendly interface, simple set up, stable performance, and tons of features. Based on the powerful PHP language and the MySQL database, XMB is a very powerful combination, suitable to everyone's needs.
Homepage: http://www.xmbforum.com/
Support Forum: http://forums.xmbforum.com/