Drupal can support a variety of websites ranging from personal weblogs to large community-driven websites
Website: Drupal.org
Community: Drupal Support Forums | Demo | Download Page
Templates: Drupal Themes | Themesbase
Plugins: Drupal Modules
Requirement: MySQL / PHP 4.2 or greater / Multi-platform
Features
Users can integrate accounts from Jabber, Blogger, and Livejournal and also with Open LDAP server. Users can customize the template to their satisfaction
Comes with polls, templates, comments can be threaded and drupal has a good version control feature. SEO friendly URLs, collaborative projects can be executed with ease, completely open source.
Drupal has web based administration with logging, reporting, analysis, tracking and statistics about visitors.
Full featured discussion forms is embedded for community based websites.
Support caching so that drupal can effectively handle slashdotting effect.