Contenta CMS is a decoupled, API-driven Drupal distribution built using the JSON API. It is being built through the same community initiative pushing forward the JSON API module. The project's home page can be found at http://www.contentacms.org/.
It provides many preconfigured options, including customizations to default endpoints. It also provides Simple OAuth to set up decoupled authentication with your frontend consumer and the API backend.
On top of delivering a distribution, the community contributors have developed various frontend consumers as examples: