Free Website Content Management Systems

Joomla
Free open-source website content management system. It can be used for everything from simple websites to complex corporate applications. Additional fancy add-on components are also available: CRM, email management, real estate management, what have you - all for a nominal price. Joomla is known to have the largest support and website template community.

Drupal
Another free open-source website content management system. Also very good. Some argue it is less user friendly than Joomla.

DotNetNuke 
It is an open-source framework (content management system) ideal for creating Enterprise Web Applications.  It is a very powerful piece of software but it does require quite a bit of learning and it does have its weaknesses.

Sourceforge.net 
This is the world's largest development and download repository of Open Source code and applications. If you have some programming knowledge, this resource would save you incredible amount of time and money. For site statistics, search for "web logs" and select from hundreds of free programs.

World Press - Open Source Blogging Platform
WordPress is a semantic personal publishing platform with a focus on aesthetics, web standards, and usability. It has a nice looking interface and offers a lot of features. WordPress is free to download.

More content management systems (CMS) from Wikipedia
An extensive list of various content management systems from Wikipedia.