Pico is an open source CMS application that is capable of very few things. It uses flat-files as the database and built with PHP. Simply, there is no setup and the app works instantly.
The contents are stored in .md files, markdown formatting is used and they can be edited in your favorite text editor. For the designs/theming, Pico makes use of the Twig templating engine and it is documented well.
Requirements: PHP 5.2.4+Website: http://pico.dev7studios.com/