Very useful page on configuring buddyPress through the wp-config equivalent bp-custom.php.
WordPress plugin that appends a wikipedia (or other mediawiki site) entry to the bottom of a wp post / page. Uses ?action=render on the end of the url to get just the HTML for the article, not the site. Useful as a resource.
Incredible little app that can run from an iPod on Linux / Mac / Windows and lets you manage your iPod. Beautiful. :-) Continuation of Yamipod.
All sorts of goodies released under the GPL from the folks behind WordPress. Including jobs, stats stuff, mediawiki login integration, wpshell, and lots more.
Plugin to automatically generate a table of content (TOC) for WordPress posts / pages.
ProjectPier is a Free, Open-Source, self-hosted PHP application for managing tasks, projects and teams through an intuitive web interface. An opensource version of basecamp! Looks pretty basic, not nearly as ajaxy / designed as 37 signals.
KMyMoney is a KDE based, open source financial accounting package. The web site looks very basic / old school.
Collabtive is an open source clone of BaseCamp (the 37 signals project management software) built in PHP backed by MySQL.
Eventum is an issue / bug tracking system developed by MySQL AB and used internally at MySQL.