Bludit - Bludit is a web application to build your own website or blog in seconds, it's completely free and open source. Markdown support.
Netlify CMS - An open-source CMS for your Git workflow
WonderCMS - WonderCMS is the smallest flat file CMS. Completely free, open source, privacy oriented for over 12 years.
Cloud Cannon - Cloud Cannon turns Dropbox/Git-project into a CMS you can setup in seconds
BoidCMS - Flat file CMS for building simple websites and blogs.
Sanity.io - Sanity.io a platform for structured content that comes with an open-source editor that you can customize with React.js.