WMForum
A very simple discussion board program written in PHP. It doesn't use any SQL database.

Bestweb
Usenet style threaded discussion, multiple forums, multiple languages, automatic archiving, printable pages, and message voting.

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

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

PHP/XML Forum
A free thread-oriented, XML-based news forum. No database required.

NeoBoard
Uses either a MySQL or PostgreSQL database. Features session support, search engine, file attachments, auto mailer, printable messages, and theme support.

Dynamic Flash Forum
A PHP and MySQL based message board with animated and easy to use Flash interface. [Open source, GPL]

MyCPForum
PHP forum script using mySQL. It is mimics the format and style of the Code Project forum. [Freeware]

WiForums
Interoperable (vBulletin, phpBB2 and IPB) Web/WAP Discussion Forums for Cell phones, PDAs and other handheld devices.

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

RedKernel VS Forum
A very secure forum script, it is protected against all know attacks and works without mySQL. [Freeware]

FunkBoard
A multi-thread customisablebulletin board messaging system. It uses PHP and either MySQL or PostgreSQL database. [Freeware]

tribbyBoard
A free threaded message board script written in PHP with a MySQL backend.

YaBB SE
A PHP/MySQL port of Yet another Bulletin Board (YaBB) 1 Gold (see above).

PunBB
Lightweight PHP discussion board with MySQL/PostgreSQL database backend

FUD Forum
Very feature-rich PHP forum system with MySQL/PostgreSQL backend

myBB
Coded in PHP and backed by mySQL. Free

ProBB
ProBB is a web bulletin board written in PHP and backed by several databases, including MySQL, MsSQL, and PostgreSQL.

QuickBB
QuickBB is a flat-file forum system built on the PHP scripting language.

WowBB
WowBB offers features such as auto install/upgrade, a WYSIWYG editor with integrated spell-checker, auto time zone detection and smart caching. Requires PHP and MySQL.

MercuryBoard
Bulletin board written in object-oriented PHP4 with a MySQL database backend.

YapBoard
A simple PHP/MySQL message board.

miniBB
Small, fast and free discussion board system written in PHP, can be based on any database supported by PHP (including mySQL and PostgreSQL).

W-Agora
Free forum written in PHP. It has also a guestbook, a download area, a photo gallery and a news publisher feature. It uses MySql as database.

ccForum
A simple, easy to use message board system. It supports single or multiple forums, user registration and anonymous posting, and forum administrators. ccForum is written in PHP and is based on a MySQL database. [Open source, GPL]

Prattle
A message board system implementing a clean linear model. [Open source, GPL]

DCForum+
A PHP/MySQL implementation of DCForum web discussion program. [Commercial]

Smart Bulletin Board
Offers all purpose software for building online interactive communities. [Freeware]

SuperThread
A discussion forum solution for web site. [Commercial]

Supermod
A PHP forum software to a higher level YABBSE based concept radically modified and customized for free.

Gravity Board X (GBX)
A PHP and MySQL based message board system. [Open source, GPL]

HydroBB
A bulletin board powered by PHP and MySQL. [Freeware]

tForum
Free threaded forum. It supports unlimited categories and boards, unlimited moderators, web-based administration and IP banning. It uses PHP and MySql.

UseBB
A PHP/MySQL bulletin board package designed for small to medium sized web sites. [Open source, GPL]

SowiBB
Forum software written in PHP with a MySQL backend.

X Forum
Basic forum script using php/MySQL.

MBoard
A simple message board for those who don't want or need large forums. Supports smileys, e-mail notification and more. [Freeware]

TinyPHPForum
An open source PHP-based database-free very small and simple forum with Multi-language, multi-skin, avatar uploading support.

bSpeak Threaded Messageboard
A PHP and MySQL based messageboard software. [Commercial]

Oxygen
A highly customisable open-source web-based bulletin board that requires PHP4, a mySQL database.

phpNet
A feature enriched PHP/MySQL message board.

DevoyBB
A free forum system (former Xtreme Boards).