
Regardless my personal opinion about it, Drupal has become one of the most (if not the most) popular CMS (Content Management Systems) around.
There are dozens of CMS, visit
cmsmatrix.org for a comprehensive list and comparison tool.
From the official Drupal website:
Drupal is a free software package that allows an individual or a community of users to easily publish, manage and organize a wide variety of content on a website. Tens of thousands of people and organizations are using Drupal to power scores of different web sites, including: community web portals, discussion sites, corporate web sites, intranet applications, personal web sites or blogs, aficionado sites, e-commerce applications, resource directories, social networking sites and more.