Drupal (Web content publishing)
Drupal is the leading open-source website creation and content collaboration tools. A modular approach to website building, from simple out-of-the-box websites to complex sites is possible with a short learning curve. Get more info on how to get started. Drupal requires an installation of a LAMP server stack; if you have not already installed LAMP, it will be installed along with Drupal 6. Drupal is available as a package from Synaptic, or from the command-line terminal:
Joomla (Web content publishing)
Joomla is a powerful open source website creation and content management tool that allows website creation for use in every arena from the simple to complex corporate environments. Info for beginners is a good place to start.
Scribus (Desktop publishing)
Scribus is an open-source package that provides professional-appearing desktop publishing.
Plone (Content Management System)
Plone is a free, open source (GPL-licensed) multi-platform content management system used by many large organizations around the world. It is available with an integrated installer here. Some users have had some difficulties in Jaunty, due to changes in Python.