Drupal (http://drupal.org) – is a free and open source modular framework written in the programming language php. Drupal allows the system administrator to organize the content, customize the presentation, automate administrative tasks, and manage site visitors and contributors. Although there is a sophisticated programming interface, most tasks can be accomplished with little or no programming.
Joomla – (http://www.joomla.org/) – for publishing content on the world wide web and intranets. The system includes features such as page caching to improve performance, RSS feeds, printable versions of pages, news flashes, blogs, polls, website searching, and language internationalization. Joomla is licensed under the GPL.
Mambo (http://mambo-foundation.org/) – for creating and managing websites through a simple web interface. It has attracted many users due to its ease of use. Mambo also includes more advanced features such as page caching to improve performance on busy sites, advanced templating techniques, and a fairly robust api. It can also automate many tasks such as web indexing for static pages.
WordPress (http://wordpress.org/) – is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability. What a mouthful. WordPress is both free and priceless at the same time. More simply, WordPress is what you use when you want to work with your blogging software, not fight it.