NuttX is a real-time operating system (RTOS) with an emphasis on standards compliance and small footprint. Scalable from 8-bit to 32-bit microcontroller environments, the primary governing standards in NuttX are Posix and ANSI standards. Additional standard APIs from Unix and other common RTOS's (such as VxWorks) are adopted for functionality not available under these standards, or for functionality that is not appropriate for deeply-embedded environments (such as fork()).

This list is an exploration of the world of UNIX®, including UNIX history, the relevance of UNIX today, and lists select awesome UNIX and UNIX-like projects. This list also contains resources for UNIX standards, programming, communities, and free software.

This is the announcement, more details can be found on https://github.com/EtchedPixels/FUZIX
Note that it is not limited to 8-bit processors: core code can be built for 6502, 6809, 68000, 8086, MSP430, pdp11 and Z80/Z180.

A listing of ITS 1.0 3/19/67 has been scanned 2023-12, see https://gunkies.org/wiki/ITS_138 and https://github.com/PDP-6/ITS-138/tree/master/ITS-Listing

A security-oriented operating system (OS).

Explanation of the possible states of a Linux process.

Good summary and collection of links to Plan 9, its various ports and descendants.

Redox is a Unix-like Operating System written in Rust, aiming to bring the innovations of Rust to a modern microkernel and full set of applications.

   Microkernel Design
   Most features are implemented in Rust
   Includes optional GUI - Orbital
   Newlib provided for C programs

   MIT Licensed
   Drivers run in Userspace
   Includes common Unix commands
   ZFS Filesystem Support (WIP)

Interesting comparison between the two operating systems.