Drupal: D is for Dynamic
Drupal is a dynamic open source content management system/web application framework originating with Dries Buytaert and underpinning at least 2% of websites globally, from personal blogs to government sites.
The system received quite a boost when it was used to build the website of a presidential candidate, Howard Dean, in 2004. After the campaign ended, members of Dean’s web team team continued their interest in the software, ultimately forming CivicSpace Labs, one of the first companies to develop and distribute the technology.
It’s written in PHP and distributed under the GNU license. 7.15 was released on 1st August 2012, and Drupal 8 is scheduled for release August 2013.
The community is extensive and very active, with conferences and camps happening all over the world, and which wouldn’t be complete without the Drupalgangers (google it).