1. Drupal.org. https://www.drupal.org
2. Joomla!.org. https://www.joomla.org
3. Magento - eCommerce Software & eCommerce Platform Solutions. https://magento.com/
4. Usage Statistics, Market Share of Content Management Systems for Websites. http://w3techs.com/technologies/overview/content_management/all
5. WordPress.org. https://wordpress.org