OpenWRT
The OpenWrt Project is a Linux-based operating system designed for embedded devices (primarily routers). Instead of being a fixed and locked operating system, OpenWrt offers a fully customizable operating system through a package manager. This allows you to break free from the applications and configuration installed by the manufacturer, enabling you to customize your device through the addition and use of packages that bring new functionalities.
For developers, OpenWrt is the ideal environment to build an application without having to build an entire firmware around it. For users, OpenWrt allows total customization, as well as the possibility of using the device for purposes never originally envisioned.