Drupal is very popular among government, many government websites using Drupal such as WhiteHouse.gov, Commerce.gov, and others. Not just among goverment, but Drupal is also very popular among enterprise, recently even Twitter uses Drupal.
A few days ago, Energy.gov also switched to using Drupal because it provides more scalable and flexible architecture compared to another, and saving taxpayers more than $10 million annually. This completes the Drupal wins in government. Congrats for the Drupal community.
You can read the announcement made by Energy.gov here, and see the new Energy.gov Drupal website here.