Joomla! is one of the most popular open source CMS. The reasons behind Joomla's popularity are its easy usability and extensibility and most of all, it's free. Written in PHP (and using MySql database), it is widely used to create content based website to even web applications. To draw your attention a little more lets say that Joomla is used by big guys like MTV, Citibank, Harvard University, L.A. Weekly etc.. 

Joomla has a huge number of extensions which provide specific functionality to Joomla sites adding to it's power and flexibility.  Installing and setting up Joomla is very easy. Before downloading and installing Joomla, you may want to get a feel of it by using the online demo. You can check out the online demo here.

For developers, there are several scopes to get involved with Joomla development. You can either contribute to the main project or built extensions. Designers can get involved in creating templates etc..

Joomla is fairly easy to get started with and you can find the official documents very useful to learn about it. Also, there are tons of Joomla tutorials on the web which you can take advantage of to get started with it or to know more about it. 

Joomla is free and easy to install and set up. Hence, if you are looking for a free CMS to build your website on, end your search today and try it out.