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).
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.
Down the 2011 Open Source CMS Market Share Report from water&stone. This is the 4th annual report of the industry's leaders, laggards and systems to watch. 62 pages. Over 30 exhibit. Lead analyst: Ric Shreves.