FreeRTOS
@ https://github.com/FreeRTOS/FreeRTOS
Popular real time operating system for embedded devices.
Genode
@ https://genode.org/about/index
Framework for building highly secure special-purpose operating systems.
L4Re operating system framework
@ https://l4re.org/overview.html
The L4Re Operating System Framework provides the building blocks for creating systems with real-time, security, safety and virtualization requirements.
UNIX odyssey
@ https://www.hypertext.download/unix-odyssey.html
Collection of resources, learning about UNIX.
OSdev wiki
@ https://wiki.osdev.org/Main_Page
Information about the creation of operating systems and community for people interested in OS dev.
MINIX from scratch
@ https://github.com/o-oconnell/minixfromscratch
Tutorial information for building MINIX, said to be a good learning OS for people interested in UNIX.
MINIX 3
@ https://minix3.org/
Official website for MINIX 3, which is the latest version.