PBLang - Bulletin Board System (BBS) written in PHP. Customisable and with many professional features, it does not require a database. - http://sourceforge.net/projects/pblang/
eForum - Web discussion forum. (Java) - http://www.jcorporate.com/html/products/eforum/eforum_descrip.html
VAXbb Project - A fast, text-based bulletin board system for the Web. (Perl) [GNU/Linux, Unix] - http://www.omar.org/opensource/vaxbb/
Phorum - Web based discussion software. (PHP) - http://phorum.org/