Metalsmith - An extremely simple, pluggable static site generator.
Grav - The modern open source flat-file CMS
Nikola - Nikola is s static site generator tool written in Python.
Bludit - Bludit is a web application to build your own website or blog in seconds, it's completely free and open source. Markdown support.
GatsbyJS - Blazing-fast static site generator for React
HTMLy - HTMLy is an open source databaseless blogging platform. A Flat-File Blog and Flat-File CMS written in PHP prioritizes simplicity and speed.