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.
OpenLaunch is a Web-based content management system that integrates a broad array of services into a single, fully-featured, elegantly designed Web site. The project aims to eliminte the difficulty of integrating multiple systems into a single Web site by implementing a blog, forum, Web pages, wiki, and (coming soon) E-Commerce and support tickets. It is built around the Torpedo Framework, an open MVC framework which includes support for controllers, forms, and a large number of other very useful utilities.