PForum is a web-based discussions application for Linux and Solaris. It offers adaptability, great look and feel customization, rapid deployment, easy administration, and language localization.
The CGIMachine - Userforum is a fast and easy to use 32-Bit application for your Windows NT-WebServer. It provides your website with a powerful forum and news area where your site visitors can leave messages and/or answer to existing messages. It features: Web-based password protected admin area, easy customization, set the number of entries per page and option to allow HTML-Code in the messages.
CxIV is a CGI (Web) Based Discussion System (written in C) tailored for Education, though it'll work just fine for other purposes. Features include: Simple Setup from Web interface, Simple Configuration from Web interface, Multiple numbered fully threaded message bases, Compile time selectable grading system, Multiple controlled user handling, Detailed Thread Tree Display System, Detailed Message Display, Internal "Instant Messaging" System and much more.