RIOT - The friendly OS for IoT

Segger RTT Assistant

Zephyr is a new generation, scalable, optimized, secure RTOS for multiple hardware architectures.

Apache NuttX Apps is a collection of tools, shells, network utilities, libraries, interpreters and can be used with the NuttX RTOS

Apache NuttX is a mature, real-time embedded operating system (RTOS)

OpenThread released by Google is an open-source implementation of the Thread networking protocol