I just had the urge to make a list of some of the biggest Open Source Content Management Systems (and some that aren’t OS) out there to be use in the PHP/MySQL development arena.

• Joomla! (probably the largest community overall – the continuation of Mambo)
• Drupal
• SilverStripe (Beautiful Admin Interface)
• CMS Made Simple
• PHP-fusion
• Alfresco (Enterprise Level CMS?)
  
• e107.0rg
• Pligg
• Xoops
• Geeklog
• NucleusCMS
  

Notable Mentions (non-CMS Open Source Projects)

• phpGEDview (Open Source Genealogy)
• CodeIgniter (PHP Code Library free to install on your webserver)