Joomla! is one of the most powerful Open Source Content Management Systems on the planet. It is used all over the world for everything from simple websites to complex corporate applications. Joomla! is easy to install, simple to manage, reliable, and extremely powerful. 

Subdreamer CMS is a powerful yet simple program used for creating websites. With it's quick installation and easy to use admin interface you will have your website up and running within minutes.

Mambo is an open-source all-inclusive content management system for creating and managing websites through a simple web interface.  It has attracted many users due to its ease of use.