From phpBB Group:
phpBB is a free flat-forum bulletin board software solution that can be used to stay in touch with a group of people or can power your entire website. With an extensive database of user-created modifications and styles database containing style and image packages to customize your board, you can create a very unique forum in minutes. Features include UTF-8 Support, XHTML 1.0 Strict Compliant, moderation, administration, search engine spider handling, unread message tracking, private message system, user logging, warnings, user banning and suspension, flood control, CAPTCHA confirmation, post approval, password hashing, database query caching, template caching, and install or manage custom styles. It supports MySQL, SQLite, Firebird, Oracle Database, Microsoft SQL Server, and PostgreSQL data management. It also supports Backup and Restore.
What's new in this version:
Version 3.0.11 has improved compatability with PHP 5.4 and added a warning about the minimum PHP version required to upgrade to phpBB 3.1 to warn users of old PHP installations.