phpBB is a popular open source bulletin board system. It is written in php and uses a database as a backend. Supported databases are: MySQL, MS-SQL, PostgreSQL and Access/ODBC.
Other features include:
unlimited forums and posts
community system: features ranks, moderation, usergroups and optional avatars.
The software itself is written and has been released by the phpBB group under the GNU General Public Licence and may be freely distributed. It is currently one of the most popular solutions for building communities, discussion and support forums.
Source: phpBB website - http://www.phpBB.org/