Drupal is a free open source content management system written in PHP, driven by a backend MySQL database. Drupal comes with a variety of easy-to-use features to build internal and external-facing websites in a matter of hours, without need of regular programming. You can also manage your content including video, text, blog, podcasts, and polls with robust user management, menu handling, real-time statistics and optional revision control. The Drupal user can be assigned one or more roles, and each role can be set up with fine-grained permissions allowing users to view and create only what the administrator permits. It also features better design themes that give interactive experiences that engage users and increase traffic. It also makes it easy to connect your website to other sites and services across the web, using aggregation, feeds, and search engine connection capabilities.
With more than 6000 modules, you can build your great websites that address all your requirements. It also comes with many tools to help you organize, structure, find and re-use your content.