UseBB is a PHP/MySQL forum package. Unlike other popular forum systems, it does not strive to have as many features as possible, but to be one of the easiest and most usable forum packages, limiting the basic feature set to only the essential. It is an ideal solution for small to medium-sized Web sites which require an easy to use forum package without the bloat, that produces standards compliant output, and is secure and easily configurable.
MyBestBB is a full-featured forum based on punBB. It adds many features, modifications, and themes to the default punBB, including RSS feeds, UTF-8, XHTML 1.0 strict compliance, sub-forums, PMs, attachments, rewrite rules, the ability for an admin to move posts, easy smilies, easy polls, many improved themes, announcements and sticky topics, an activity barometer, a URL rewriting module, and much more.
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.
sfMaraePlugin is a threaded forum plugin for symfony. Current features include fully threaded categories and posts; a customizable permissions system for groups and individual users, allowing fine-grained settings for starting new topics, replying, editing, deleting, and viewing whole categories; and the ability to vote individual comments up and down.