Micro T-Kernel

μT-Kernel is an open source real-time [operating system] designed for 16- and 8-bit microcontrollers. "μ” in the name stands for "micro" and pronounced as such. It is not pronounced as "mu". It is freely available under T-License.
Supported CPU list is available.
The latest version, μT-Kernel 3.0, is available from GitHub.
μT-Kernel was standardized by T-Engine Forum and later it became the basis of IEEE Standard 2050-2018, "IEEE Standard for a Real-Time Operating System for Small-Scale Embedded Systems" published by the Institute of Electrical and Electronics Engineers Standards Association.
Its specification is available both in English and Japanese. The source code is available from the TRON Forum website and GitHub.
An article comparing nine RTOSs in which μT-Kernel was evaluated and given favorable remarks appeared in IEEE publication.

History

μT-Kernel was developed as a smaller subset of T-Kernel, a full-featured real-time operating system. For example, it does not assume the use of MMU unlike the original T-Kernel. For more on its history and the overall philosophy behind the TRON real-time OS family, please see the entry of T-Kernel.