A community software package with themes, fast message database, secure file-attaching system, and mod installation manager. [Freeware]

Template-driven PHP/MySQL forum software developed by the creators of WordPress. [Open source, GPL]

News, information, downloads, newsletter subscription, international support, hosting and forums for the lightweight discussion board. [Open source, GPL]

Aims to be a fast and lightweight PHP-powered discussion board. Licensed under GNU GPL.

Threaded or linear viewing of posts, address books, CSS compatible, session or cookie based, moderators, multi-language support, private forums, profanity filter, and user pictures. [Commercial]

PHP/MySQL forum software with multilingual support and more than 100 plug-ins. [Open source, GPL]

A free threaded, web-based bulletin board system written in PHP with a variety of backend database option available.

Forum software forked by four developers of PunBB that supports MySQL, PostgreSQL and SQLite and produces valid XHTML [Open source, GPL]

PHP/MySQL forum software that supports themes, BB code, and instant messaging. [Open source, GPL]

Ikonboard's PHP version. It supports all CGI version's features and it's free. A free registration is required to download the script.