Operating Systems: Timeline and Family Tree via Reddit - Operating Systems: Timeline and Family Tree

Visopsys (dead link)
MonaOS
HelenOS uses its own microkernel written from scratch and supports SMP, multitasking and multithreading on both 32-bit and 64-bit, little-endian and big-endian processor architectures, among which are AMD64/EM64T (x86-64), ARM, IA-32, IA-64 (Itanium), 32-bit MIPS, 32-bit PowerPC and SPARC V9.
ChibiOS/RT is a complete, portable, open source, compact and extremely fast Real Time Operating System.
NitrOS-9 is a real-time, process-based, multitasking, multi-user, Unix-like operating system for the TRS-80 Color Computer, Radio Shack Color Computer 2, Tandy Color Computer 3 and Dragon 64. The original OS-9 was created in 1979. NitrOS-9 is the modern equivalent of that OS, and includes advanced features like support for up to 2 MB RAM and 4 GB Hard drive partitions. It is still being developed, and support is available in many mailing lists and forums.
Akaros is an open source, GPL-licensed operating system for manycore architectures. Our goal is to provide support for parallel and high-performance applications and to scale to a large number of cores.
Prex is an open source, royalty-free, real-time operating system for embedded systems. It is designed and implemented for resource-constrained systems that require predictable timing behavior. The highly portable code of Prex is written in 100% ANSI C based on traditional microkernel architecture.
The OKL4 Microvisor is designed from the ground up as a high-performance mobile virtualization platform. It is a microkernel-based embedded hypervisor - called a Microvisor, with a small footprint and the right combination of performance and hardware support to target mobile telephony use.
Contiki (license: 3-clause BSD-style license)
The Muen Separation Kernel is the world’s first Open Source microkernel that has been formally proven to contain no runtime errors at the source code level.
Xv6, a simple Unix-like teaching operating system.
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()).
Minoca OS is an open source, general purpose operating system written from scratch. It aims to be lean, maintainable, modular, and compatible with existing software. It features a POSIX-like interface towards application software and a growing suite of popular packages already built and ready to go. On the backend, it contains a powerful driver model between device drivers and the kernel. The driver model enables drivers to be written in a forward compatible manner, so that kernel level components can be upgraded without requiring a recompilation of all device drivers.

List of operating systems

MenuetOS is an Operating System in development for the PC written entirely in 32/64 bit assembly language.
DexOS is a 32bit, asm OS, based on the idea, that it would be cool to have a OS based on a console type OS. Dead link. See OS Archive - DexOS.

KolibriOS is an open source operating system with a monolithic preemptive kernel, video drivers, for 32-bit x86 architecture computers, developed and maintained by The KolibriOS Project Team. KolibriOS is a fork of MenuetOS, written entirely in FASM (assembly language). However, C, C++, Free Pascal, Forth, among other high-level languages and compilers, can also be used in user application development. English, Russian and German versions are available.

BareMetal is a 64-bit OS for x86-64 based computers. The OS is written entirely in Assembly, while applications can be written in Assembly or C/C++. Dead link. See GitHub - ReturnInfinity and Return Infinity.

10 amazingly alternative operating systems and what they could mean for the future

wlindley.com - Heritage Products some MS-DOS, HDOS and CP/M software.