Grav - The modern open source flat-file CMS
Sitecake - Drag and drop CMS for HTML websites. It's flat file CMS so it's pretty fast.
Bludit - Bludit is a web application to build your own website or blog in seconds, it's completely free and open source. Markdown support.
Anchor CMS - Free and lightweight blogging system
HTMLy - HTMLy is an open source databaseless blogging platform. A Flat-File Blog and Flat-File CMS written in PHP prioritizes simplicity and speed.
Textpattern - Textpattern is an elegant content management system that is free, open source software.