This report from idealWare compares 4 popular open source content management systems (CMS) that non profits like to use. If you ever are in the market to try Joomla, Wordpress, Drupal or Plone this is the report for you.
The 2 second summary? Joomla and Wordpress are best for simple page-based sites, while Drupal and Plone are best for more complex, content-rich, multi-user websites. I would love to hear more from the community whether they agree with the report's conclusions.