Open Source CMS software enables you to participate in the development of the content management system while the use of it is totally free (also make sure to read and understand the license, though). Most [w:open source] CMS software is based on the linux operating system and free development platforms like php or python.

Screenshots:

Mambo
Platforms	Linux/BSD/MacOS/Windows
Software	PHP/MySql
Website	visit Mambo website
Download	download (Windows/MacOS/Linux/BSD)
Description	Mambo is one of the most powerful Open Source Content Management Systems on the planet. It is used all over the world for everything from simple websites to complex corporate applications. Mambo is easy to install, simple to manage, and reliable.

Plone
Platforms	Linux/BSD/MacOS/Windows/Solaris
Software	Included in install
Website	visit Plone website
Download	download installer/packages(Windows/Linux/MacOSX/BSD/Solaris)
Description	Plone is an out-of-the-box ready content management system that is built on the powerful and free Zope Application server. It requires minimal effort to set up, is deeply flexible, and provides you with a system for managing web content that is ideal for project groups, communities and intranets.