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.
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).
Recently I've been helping a few folks migrate websites from WordPress to Drupal. Presently this works for migrating to Drupal 5 so if you want a Drupal 6 site first migrate to Drupal 5 and then upgrade the site to Drupal 6.
CiviCRM is an open source and freely downloadable constituent relationship management solution. CiviCRM is web-based, open source, internationalized, and designed specifically to meet the needs of advocacy, non-profit and non-governmental groups.