Drupal

Drupal is a free online Open Source web content management system platform. Drupal can be used to create a wide array of webpages, from personal blogs to enterprise applications. The nature of the open source community surrounding the Drupal CMS allows users to access extra CMS features (called modules) that were created by developers in the Drupal community, as well as contribute to the array of features themselves by utilizing a Drupal developer account.

As of October 12th, 2012, there were 18,627 modules, 21,153 developers, and 886,121 users worldwide using Drupal to power their sites.