Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
While Drupal isn't the most popular open source content management system (CMS) available today, its focus on deep management and customization features ideal for complex sites has led it to be used on some very large and popular sites, including many busy news sites and WhiteHouse.gov.
But when it comes to ease of use and flexibility, Drupal has lagged behind more popular competitors such as Wordpress. However, with the release of Drupal 7, the open source CMS has significantly overhauled much of the core of the product and has taken some significant strides towards becoming a more user friendly -- but still powerful -- CMS.
Installing Drupal 7 is fairly painless and the CMS can run on any system that can run PHP and MySQL, which includes both Linux and Windows systems. Upgrading from version 6 could be a difficult task, given the many changes in the core of this release, especially if your site has a lot of custom code and configurations.