You're probably familiar with Twitter; what you may not be familiar with is their extensive additional content hosted outside of their main application. Through various projects via Phase2, I worked in Twitter's custom-built Drupal framework (codenamed Gazebo), to build out various help, support, and information sites, ready for their support and sales teams to create content in. They have since moved to using Adobe Experience Manager, and all that remains of Gazebo, as far as I know, is these screenshots.
One particularly interesting feature of Gazebo was an implementation of Twig templating in Drupal 7, while, at the time, core Drupal was still using the PHP templating engine. I used the knowledge gained there to do a similar implementation on later projects, before transitioning fully to Drupal 8's Twig templating system.