fp.board is a bulletin board software solution designed to both implement features of proprietary non-free solutions and provide exceptional security. It supports features such as a built in Markdown Engine along with BBCode, an advanced Ban Control Panel (modeled after IRC, with Account Exceptions and CIDR based IP bans), DNS Blacklist checks, checks against the StopForumSpam databases, and more. It has no issues with the Suhosin Patches, and is developed on a server that supports Suhosin.
Sijio is a fast, high performance, easy to use and fully customizable PHP community script. It is equipped with all necessary professional tools and features to run a full fledged community site. Features include blogs, events, a forum, a gallery, polls, page management, user management, and RSS feeds, customizable Smarty powered templates, WYSIWYG editing of any content, easy picture upload, an unlimited number of attachments, and more. It is completely SEO friendly.
FluxBB is a fast, light, user-friendly forum application for Web sites. It was originally based on PunBB and supports UTF-8, XHTML, and internationalization, and leaves out some of the less than essential features of other forums without sacrificing essential functionality or usability.
Nearforums is a lightweight powerful discussion forum that can be installed within minutes on Linux or Windows servers with MySQL or MS SQL database servers. It includes great features like sign-in with Facebook/Twitter, navigation with tags, full Web administration, built-in search engine, RSS feeds, and accessible HTML. It is SEO friendly.
TinyIB is a lightweight PHP image board which emulates the functionality of 4chan. If you use MySQL or SQLite, you can use it to create an efficient setup able to handle large amounts of traffic. If you don't use a database, it can store posts as text files for a portable setup capable of running on virtually any PHP host.