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.
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.