WordPress is a free and open-source content management system based on PHP and MySQL. Features include a plugin architecture and a template system. It is most associated with blogging but supports other types of web content.
Joomla! is a free and open-source content management system for publishing web content, developed 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 provides a back-end framework for at least 2.3% of all web sites worldwide – 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