According to the 2009 Open Source CMS Market Share Report by CMSWire and Water & Stone, "Joomla! is the web’s most popular open source content management system".
Twenty open source web content management systems are covered in the report with WordPress, Joomla! and Drupal leading the others by a significant margin.
Emerging as the leader, Joomla! now leads WordPress in current usage by more than 6.5 percentage points and leads Drupal by almost 7 points.
Download the full report for free from CMSWire.