FreeRTOS

Richard Barry wrote it around 2003. The kernel is a handful of C files — scheduler, queues, semaphores, software timers — and needs only a few kilobytes of flash. Porting it to a new CPU is mostly a matter of writing the context switch in assembly, which explains why ports exist for more than forty architectures.

AWS took over stewardship in 2017 and swapped the modified GPL licence for MIT, which made commercial use painless. Espressif's ESP-IDF is built on it, so every ESP32 project runs FreeRTOS whether the programmer knows it or not. The kernel deliberately does little: networking, file systems and drivers are add-ons. That is both its strength and the reason the Zephyr Project has caught on with people who want everything in one box.