Drupal is becoming one of the most popular, if not the most popular true content management systems as this point. Of course it is not on par with Wordpress in terms of usage by bloggers, but it has a tremendous amount of functionality for an open-source CMS platform. The development community has really come through as well as there are tons of modules, otherwise known as plugins to the rest of the world, available to add much more functionality to your Drupal site.

Currently the Whitehouse.gov runs on Drupal, along with all of my websites.