WordPress is web software you can use to create a beautiful website, blog, or app. We like to say that WordPress is both free and priceless at the same time.
WordPress probably has the widest base of plugins and themes to choose from. A great part about the WordPress community is the amount of help and documentation online you can find on nearly every aspect of customizing WordPress. If you can dream it, chances are it’s already been done with WordPress and documented somewhere.
Drupal is another CMS that has a very large, active community. Instead of focusing on blogging as a platform, Drupal is more of a pure CMS.
Drupal is an open source content management platform powering millions of websites and applications. It’s built, used, and supported by an active and diverse community of people around the world.
Drupal helps you build everything from personal blogs to enterprise applications. Thousands of add-on modules and designs let you build any site you can imagine.
Joomla is a very advanced CMS in terms of functionality
Joomla! is an award-winning content management system (CMS), which enables you to build Web sites and powerful online applications. Many aspects, including its ease-of-use and extensibility, have made Joomla! the most popular Web site software available.
Chamilo is an open-source (under GNU/GPL licensing) e-learning and content management system, aimed at improving access to education and knowledge globally. The Chamilo project aims at ensuring the availability and quality of education at a reduced cost