Bludit - Bludit is a web application to build your own website or blog in seconds, it's completely free and open source. Markdown support.
PhileCMS - A file-driven CMS powered by Markdown and Twig.
Monstra CMS - Fast and small flat-file content management system written in PHP!
Vapid - An intentionally simple 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.
BoidCMS - Flat file CMS for building simple websites and blogs.