Directus is an open-source "headless" CMS & API for managing custom databases.
"Content Management Systems" should manage content. That's it. They should do that job perfectly and then get out of the way. Directus does just that. It allows you to author content independent of where it will be used – this decoupling is why it's called a "headless" CMS.
Content is managed directly in your custom database – with no assumptions about your project, no limitations on architecture, and no proprietary storage. All your data is accessible through the Directus API and SDKs – or, for self-hosted projects, you can pull right from the database. Now you have the freedom to connect any number of: websites, native apps, wearables, kiosks, internet-of-things devices... any data-driven projects.
Simple and intuitive for small projects, performant and scalable for enterprise – Directus is a flexible framework that your project won't outgrow. All this, and Directus is still completely free and open-source.