PhpBB

phpBB is a popular web-based forum package, written in open-source PHP. It is used by quite a few websites, large and small.

Links

 * Official site
 * Wikipedia article
 * What is wrong with PHPBB? (discussion)
 * Development wiki
 * Bug tracker
 * Source code
 * API docs (3.0.x)
 * API docs (3.1.x)