Great summaries:
- https://en.wikipedia.org/wiki/UNIX_System_V#/media/File:Unix_history-simple.svg
- https://en.wikipedia.org/wiki/Usage_share_of_operating_systems#Market_share_by_category
Major alternatives:
- https://en.wikipedia.org/wiki/IBM_AIX, IBM system V based
- https://en.wikipedia.org/wiki/HP-UX, HP system V based
Strict real time:
- https://en.wikipedia.org/wiki/QNX, proprietary, micro-kernel, bought by BlackBerry.
- https://en.wikipedia.org/wiki/VxWorks, proprietary
- https://en.wikipedia.org/wiki/ECos GPL, started by Cygnus, discontinued by Red Hat, copyright later transferred to the FSF.
- https://en.wikipedia.org/wiki/FreeRTOS
- https://en.wikipedia.org/wiki/Micro-Controller_Operating_Systems proprietary with source available
https://en.wikipedia.org/wiki/Xenomai provides real time capabilities on Linux userland.
Dedicated OSes / firmware:
- https://en.wikipedia.org/wiki/Rockbox , music players
Microcontrollers: seems like Linux requires a MMU: