Grav is a modern and open-source Flat-File-CMS. Grav has been created by Andy Miller from RocketTheme and it quickly became popular in the community. One of its specials: Grav has its own package manager.
HTMLy is a lightweight, flat-file blog software that prioritizes simplicity and speed. If you plan a simple, standard-blog, for example, to accompany your next web project, then HTMLy is probably a good choice.
Kirby is a well established, professional flat-file CMS with a great continuity. Kirby was first published in 2009, but, due to the continuous development, it is as fresh and modern as its juvenile competitors.
Statamic is a premium Flat-File-CMS with a modern code base and an extensive set of functionalities. You can create simple blogs as well as websites of mid-range complexity. All without the need for a database.