Zephyr (RTOS)
The Linux Foundation's real-time operating system for microcontrollers, built with the Linux kernel's tooling and the Linux kernel's ambitions.
Launched in 2016, it started from Rocket, a small kernel Wind River donated to the effort. It is Apache 2.0 licensed and governed by a member organisation that includes Intel, Nordic Semiconductor, NXP and Google — an RTOS run as an industry consortium rather than a weekend project.
What separates Zephyr from FreeRTOS is scope. FreeRTOS is a scheduler and a few queues; Zephyr ships drivers, a networking stack, Bluetooth, filesystems and a bootloader in MCUboot. Configuration goes through Kconfig and hardware is described in devicetree, exactly as in Linux. The upside is that one application builds for hundreds of boards. The downside is that a newcomer can spend an afternoon working out why a single pin refuses to exist.