Drupal

Drupal is a free and open source software application that is installed on servers. When installed and configured, Drupal helps people make websites that are secure, safe, and stable. Drupal also allows users to share modules with each other. Modules are like building blocks that when installed make new features available.
As of February 2016, more than 1.3 million websites use Drupal.
Drupal is well supported, and many high level websites are built with it including whitehouse.gov.