Elgg.org
Elgg is a highly customizable web framework and CMS for building social apps with PHP and MySQL
Plasmic
Plasmic is a visual, no-code page builder and CMS for any website or codebase. Use your existing code components. Empower the whole team to ship incredibly fast.
KeystoneJS
Build faster and scale further with the programmable open source GraphQL API back-end for structured content projects.
Joomla CMS
Source-code: https://github.com/joomla/joomla-cms More: https://framework.joomla.org/ Extensions: https://extensions.joomla.org/