Drupal is a powerful content management system (CMS). CMSs enable easy management of workflow in a collaborative environment to power any type of website, from government sites to social networking sites.
Although Drupal offers a sophisticated programming interface for developers, no programming skills are required for basic website installation and administration.
Video: http://dai.ly/gabKRV
For a full list of governments using drupal see:
http://groups.drupal.org/node/24119#USA
http://groups.drupal.org/government-sites#USA
Key features
Excellent documentation and community with a highly active forum
Handles countless content types including video, text, blog, podcasts, and polls with robust user management, menu handling, real-time statistics and optional revision control
Focus on social publishing can help you create sites and applications to help your users express their opinions and engage with one another
Build internal and external-facing websites in a matter of hours, with no custom programming
Lots of add-on modules to extend functionality