I know I'm a week behind about this, but I just came across the link via Gadgetopia:  Open-source PHP applications that changed the world.  The CMS related applications that were determined world changers are listed below.

• PHP-Nuke - 1999
• eZ Publish - 2000
• osCommerce - 2000
• phpBB - 2000
• Gallery - 2001
• Drupal - 2001
• MediaWiki - 2002
• WordPress - 2003
• Joomla! - 2005
• Magento eCommerce - 2007

When you split the list into three sections, you get the feeling that PHP-Nuke, ezPublish, and osCommerce have been around for a very long time.  More importantly, you begin to think that it is time for Gallery, Drupal, and MediaWiki to either be seen as mature applications or for those applications to finally grow up.