b2evolution is a Content Management System aimed at making it easy to build and maintain websites for sharing information and collaborating with your community.
From a simple homepage to a blog, a photo gallery or a newsletter... all the way up to a full-featured community site with forums, members directory and private messaging, b2evolution includes everything you need in a single integrated package (which saves you time on setup and upgrades).
b2evolution is Free Open-Source software that you can download and run on the web-hosting service of your choice.
b2evolution can be used as a headless CMS through its Restful API: https://b2evolution.net/man/advanced-topics/external-apis/rest-api/
See demo here: http://demo.b2evolution.net
Find more headless content management systems.