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.
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.
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 powerful, easy to use utility to allow building small-ish (dozens to hundreds of pages), websites and web apps.
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.
Website Baker 2 features include a template-based front-end, multi-level modulated page support, multi-user administration, and much much more!
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.
e107 is a free (open-source) content management system built for bootstrap 3 and running on PHP/MySQL.