Open-source CMS for building websites
WordPress: It’s the leader. 27% of all the websites of the world are built with Wordpress, and 58% of all the websites that use a CMS. It surely offers the largest community and list of additional features. However it is not really loved by developers as its code and structure are not really excellent and it is based on an old version of Php. These points don’t make it really recommended for big and complex websites.
Joomla: it is a very popular CMS, even if its market share is really low, 3.4% of all websites, compared to WordPress, but WordPress is a freak in terms of market shares :). Originally Joomla was really performing for its system of page organization. It is a bit more complicated to master than WordPress but it is also more modern in terms of code, and so maybe more efficient for big websites.
Drupal: Drupal market share is 2.2% of all websites, it is a good CMS but can be quite complicated to understand. It has a large community and many plugins.
Magento: This is the leading open-source CMS for ecommerce. It is a really complete and efficient solution for building either small or big ecommerce websites, finally more big than small ones though. Its relative smaller market share can be explained by this “ecommerce” specialization, it is not really used for other things than ecommerce websites.
Open-source CMS for building mobile applications
Siberian CMS: open-source CMS for building mobile applications are not so common and younger than CMS for websites, but Siberian CMS can be seen as the leader. It has grown fast and is offering a strong solution and many modules on its marketplace. The community around Siberian CMS is quickly growing too. It is based on Php language for the admin side and on Angularjs language for the mobile applications (iOS and Android). The fact that it is based on web languages makes this CMS quite popular among developers, and it is very used by communication agencies and consultants without technical knowledge all over the world.
Keosu: It is a french based CMS. It is lightweight and smart but quite simple with not so many features. However it is simple for developers and quite simple for non-technical users too. There is just a small community behind.