WordPress is a free and open source blogging tool and a content management system (CMS) based on PHP and MySQL.
Joomla is a free and open-source content management system (CMS) for publishing web content. It is built on a model–view–controller web application framework that can be used independently of the CMS.
Drupal is a free and open-source content-management framework written in PHP and distributed under the GNU General Public License.
Umbraco is an open source content management system (CMS) platform for publishing content on the World Wide Web and intranets. It is written in C# and deployed on Microsoft based infrastructure.