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!