Arduino (yes I know it isn't really an OS, but still), Zephyr, Oberon, Active Oberon, Inferno, mBed, Android and Chrome OS (Linux kernel isn't really exposed to userspace as in an UNIX system), Azure RTOS.

Micropython is really neat as an embedded OS. I don't think there's a PC target right now though.

Kolibri also exists, not sure how alive it is nowadays, though.

I believe Contiki is one. It runs on pretty much anything.

There's also FreeRTOS if you include microprocessors.

SkyOS: https://youtu.be/4LOCyT12X0Y

SkyOS isn't FOSS

I don't think it is maintained anymore, but add AtheOS/Syllable to the list.

TempleOS