Drupal is a free open source software modular content management system and blogging engine.
From the announcement page:
After 8 months of development we are ready to release Drupal 5.0 to the world. Today is also Drupal's 6th birthday, so the timing could not be more perfect. Drupal 4.0 was released in 2002 and finally we feel confident to increase the major version number from 4 to 5.