A free (libre), open-source (GNU GPL), content management platform designed for group collaboration. Open Atrium is based on Drupal (running on Apache, with a MySQL database).
Open-source server application packages for deployment as appliances (e.g., virtual machines) or sandboxed local installation on Linux, OS X, or Windows. Smaller application downloads are available to pair with LAMP, LAPP, and Ruby base stacks. Handy for test-driving applications.
A free (libre), open-source (GNU GPL) blogging and content-management system. WordPress is written in PHP, supports severeal operating-systems, and requires a MySQL database.
Notable WordPress sites include the U.S. Library of Congress, MSNBC, and The Rolling Stones.
A free (libre), open-source (GNU GPL) content management framework written in PHP with support for a number of operating-systems, web-servers and databases.
Drupal made international news following the U.S. White House's October 2009 web redesign. The variety of Drupal-powered websites demonstrates the platform's flexibility. Notable examples include British Medical Journal (BMJ); Stanford Law School, and Turner Broadcasting.