eForum: Web discussion forum. (Java)
Mailman: Software for managing electronic mail discussion lists. (Python)
PBLang: Bulletin Board System (BBS) written in PHP. Customisable and with many professional features, it does not require a database.
Phorum: Web based discussion software. (PHP)
VAXbb Project: A fast, text-based bulletin board system for the Web. (Perl) [GNU/Linux, Unix]