WordPress is a free content management system created by Automattic, LLC. It is a blogging platform that uses a model similar to Blogger.com. It is open-source software licensed under GNU GPL version 2.0. WordPress is written in PHP and MySQL.
Joomla! is a free content management framework originally designed for use on the Joomla CMS. It is released under the GNU General Public License. It is a powerful content management system that provides a robust set of features including user authentication, site security, search engine optimization, and much more.
Drupal is a free and open-source Content Management System (CMS). It is a flexible, scalable, and secure solution for building community sites and intranets. It is released under GNU/GPL license.
5. Expression Engine
ExpressionEngine is a free content management platform built on top of the popular Movable Type open source publishing platform. It is based on the concept of templates and includes many useful features such as blogs, forums, wikis, galleries, polls, file attachments, RSS feeds, and more.
Typo3 is a free content management solution that is based on the LAMP architecture. It is a fully featured content management system that offers a rich feature set including user authentication, site administration, news, events, blogs, forums, ecommerce, and more.
SilverStripe is a free content management suite that is based on the Symfony framework. It is a full-featured content management system that includes a blog, forum, gallery, wiki, document library, event calendar, and more.