GraphCMS - The GraphQL Headless CMS
Netlify CMS - An open-source CMS for your Git workflow
DatoCMS - Connect DatoCMS to your favorite site generator, build the perfect backend and deploy anywhere you like.
Grav - The modern open source flat-file CMS
Pagekit - A modern open source CMS. Intuitive. Modular. Flexible.
ProcessWire - ProcessWire is a PHP5-based open source CMS/CMF built to save you time and make development fun at any scale. It works the way you do.