Nucleus CMS

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TYPO3

ButterCMS is a commercial headless content management system and content API. With ButterCMS you can quickly setup your very own headless CMS that supports In-browser editing. Where you can quickly find and filter your images and files, update and schedule content and serve you assets over a content delivery network.

Quanta is a free, open source Content Management Framework thought for programmers and used for the creation of new-generation websites and applications.

Exponent CMS is a notable free and open source content management system that contains its own independent modular software framework of enterprise level.

Joomla is a free and open-source content management system written in PHP. The CMS is built entirely by volunteers from all over the world and has a strong community that provide educational resources, and extensions. 

WordPress powers roughly 30% of all web pages online. It's a free and open source CMS designed and developed for self-hosted publishing by Automattic, Inc. The software is written in PHP and utilize either MySQL or MariaDB as a database, and is compatible with most web hosts. Wordpress includes a plugin architecture and a templating system that can be used to extend and customize your website. These are commonly known as WordPress plugins and WordPress themes and make up a whole open source and commercial eco-system where anyone is free to create products for the platform. 

October is a free, open-source and self-hosted content management system written in PHP on top of the framework Laravel. The CMS features a universal extendable dashboard, inspector, reusable CMS components and a plugin system that can be used to extend functinality of your website. The plugin store contains thousands of community-made plugins that can either be purchased or installed for free. As a data store, October CMS supports MySQL, PostgreSQL and SQLite.