Bludit - Bludit is a web application to build your own website or blog in seconds, it's completely free and open source. Markdown support.
WonderCMS - WonderCMS is the smallest flat file CMS. Completely free, open source, privacy oriented for over 12 years.
Monstra CMS - Fast and small flat-file content management system written in PHP!
Grav - The modern open source flat-file CMS
WordPress - WordPress is web software you can use to create a beautiful website or blog. We like to say that WordPress is both free and priceless at the same time.
HTMLy - HTMLy is an open source databaseless blogging platform. A Flat-File Blog and Flat-File CMS written in PHP prioritizes simplicity and speed.