Directus

vs

Nucleus CMS

Strapi is a free and open source headless CMS developed with JavaScript for Node.js. The software makes it really simple to generate REST and GraphQL APIs. Simply create your database schema and relationships in the Strapi admin panel UI, and Strapi will auto-generate API endpoints to represent the data structures provided. Strapi comes packed with features you come to expect from normal CMS's such as authentication, authorization, content management and a media library. The software can be used for free and can be self-hosted on any server and platform. Extra enterpise functionallity is currently being developed, and will be sold at cost.

Ghost is a free and open-source headless CMS and blogging platform developed by Ghost Foundation. With Ghost you can manage content in style, the user experience provided by the software is intuitive and encourages writing. Ghost is written in JavaScript for the Node.js platform and is released under the MIT license.

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. 

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.