Flextype - Flat-file content management system in PHP
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.
Monstra CMS - Fast and small flat-file content management system written in PHP!
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.