Batflat - Lightweight, fast, easy, and free CMS.
Bludit - Bludit is a web application to build your own website or blog in seconds, it's completely free and open source. Markdown support.
Shopping feed - Shopping feed is a versatile eCommerce application that allows eCommerce stores to thousands of channels.
Grav - The modern open source flat-file CMS
ImpressPages CMS - ImpressPages is a PHP framework with admin panel.
HTMLy - HTMLy is an open source databaseless blogging platform. A Flat-File Blog and Flat-File CMS written in PHP prioritizes simplicity and speed.