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Microsoft ThreadX is open source.

I guess minix is kinda in that space too.

FreeRTOS has already been mentioned, but there's also NuttX/Vela.

Micrium (uC/os-ii, uC/os-iii) is open source now. Best dead-trees book for an RTOS. I haven't shipped anything on it, so I have no other opinion.

Isn't contiki-ng another one?