From the website:

  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, 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.

WWW: http://www.phpbb.com/