Drupal is a free software package that allows anyone to easily publish, manage and organize a wide variety of content on a website. Hundreds of thousands of people and organizations are using Drupal to power an endless variety of sites.
Joomla! is one of the most powerful Open Source Content Management Systems on the planet. It is used all over the world for everything from simple websites to complex corporate applications. Joomla! is easy to install, simple to manage, and reliable.
PHP-Fusion is a light-weight open-source content management system (CMS) written in PHP4. It uses a mySQL database to store your site content and includes a simple yet comprehensive adminstration system.
CMS Made Simple is an open source (GPL) package, built using PHP that provides website developers with a powerful, easy to use utility to allow building small-ish (dozens to hundreds of pages), websites and web apps.
MyBB is a powerful, efficient and free forum package developed in PHP and MySQL. MyBB has been designed with the end users in mind, you and your subscribers. Full control over your discussion system is presented right at the tip of your fingers.
WordPress is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability. More simply, WordPress is what you use when you want to work with your website software, not fight it.