WordPress is a free and open-source content management system based on PHP & MySQL. Features include a plugin architecture and a template system. It is most associated with blogging.
Joomla! An open-source content management system for publishing web content, by Open Source Matters, Inc. 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 provides a framework for at least 2.3% of web sites – ranging from personal blogs to corporate, political, and government sites.
Magento is an open-source e-commerce platform written in PHP. The software was originally developed by Varien, Inc, a US private company headquartered in Culver City, California, with assistance from volunteers.