A content management system (CMS) is a computer application used to create, edit, manage, and publish content in a consistently organized fashion. CMSs are frequently used for storing, controlling, versioning, and publishing industry-specific documentation such as news articles, operators' manuals, technical manuals, sales guides, and marketing brochures. The content managed may include computer files, image media, audio files, video files, electronic documents, and Web content.
http://en.wikipedia.org/wiki/Content_management_system

Drupal is a free software package that allows anyone to easily publish, manage and organize a wide variety of content on a website. Hundreds of thousands of people and organizations are using Drupal to power an endless variety of sites.

Redaxscript is an ultra lightweight CMS, simplifying the world of complex online projects. Experience the future of web design and development - leave behind the confusion and build websites intuitively.

concrete5 is simple for site owners to use, and powerful for site developers to work with.
You owe it to yourself to check out C5.

Joomla! 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. Joomla! is easy to install, simple to manage, and reliable.

PHP-Fusion is a light-weight open-source content management system (CMS) written in PHP4. It uses a mySQL database to store your site content and includes a simple yet comprehensive adminstration system.

CMS Made Simple is an open source (GPL) package, built using PHP that provides website developers with a simple, easy to use utility to allow building small-ish (dozens to hundreds of pages), semi-static websites.

NukeViet is the 1st opensource CMS in Vietnam. Next generation of NukeViet, version 3.0 coding ground up. Support newest web technology, include xHTML, CSS 3, XTemplate, jQuery, AJAX...

Increase engagement with customers and users. Deliver rich & compelling digital experiences. Push content to any channel, any device, at any time. Introducing pimcore the most powerful multi-channel experience and engagement management platform.

MODx is an open source CMS that helps you take control of your website. It makes updates easy, empowering end-users with as much control as you desire over website content and update frequency.

Mambo is a full-featured content management system that can be used for everything from simple websites to complex corporate applications. Continue reading for a detailed feature list.

Website Baker 2 features include a template-based front-end, multi-level modulated page support, multi-user administration, and much much more!

synType CMS is a free open source Web Content Management System. synType CMS is fast, secure, stable and easy to learn. It provides many tightly integrated features out of the box, where other systems require external modules you may even have to pay for.

WordPress is a state-of-the-art semantic personal publishing platform with a focus on aesthetics, web standards, and usability. More simply, WordPress is what you use when you want to work with your website software, not fight it.

Jaws is a Framework and Content Management System for building dynamic web sites. It aims to be User Friendly giving ease of use and lots of ways to customize web sites, but at the same time is Developer Friendly.

Tiki is a full-featured, tightly integrated, open source, multilingual, all-in-one Wiki+CMS+Groupware, written in PHP and actively developed by a very large international community.

Contao Open Source CMS (fka TYPOlight) is a powerful web content management system for people who want a professional internet presence that is easy to maintain. It was developed by Leo Feyer in 2006 and has been released under GNU/LGPL.

e107 is a free (open-source) content management system allowing you to easily manage and publish your content. Developers can save time in building websites and powerful online applications. Users can avoid programming completely! Built with PHP & MYSQL

LEPTON CMS is in fact what WebsiteBaker originally was meant to be:
easy to install, use and maintain, and at the same time be secure, flexible and powerful.
Upgrade script from WB to LEPTON is available

Kryn.cms is a revolutionary redefinition of content-managment-systems. Working with Kryn.cms is like working with your computer. The desktop environment gives you a familiar feeling from the moment you start working with it.

web@all is a PHP content management system (CMS). If you get to know it well enough, you can use it to do anything with it.

Elxis is a Free Open Source Content Management System (CMS). Elxis gives you the freedom and the power to do what ever you have in mind, without limitations or dependences.

TYPO3 is a free Open Source content management system for enterprise purposes on the web and in intranets. It offers full flexibility and extendability while featuring an accomplished set of ready-made interfaces, functions and modules.

Exponent is a web application that allows site owners to easily create and manage dynamic websites. Unlike other site management tools, Exponent is completely self-contained.

The Etomite Content Management software is released under the GNU license scheme. This means that you don't pay a penny to download, install or use it.
There are no hidden-catches, no banners, and no spyware included.

bitweaver is an application framework for content management. It is a fully functional web application and CMS, and is used to power every page on this site.

sNews is a completely free PHP and MySQL driven tool for publishing and maintaining news articles on a website. Integrating sNews into your existing
design is simple, but you could also use sNews on it's own, as a simple Content Management System.