Drupal
The enterprise OpenSource content management system which is available for free at https://www.drupal.org where you can find all the details and also references.
Drupal has a very active community with tens of thousands of developers all around the world and they also have a huge focus on standard compliance and on security.
Almost all websites have one requirement in common: getting notified when something specific is happening. Whether that's a new blog post, a new comment, a modified version of a node, an exception in the logs or a deleted user account, either the site admin or website visitors would like to know about some or all of these.