This list contains awesome OS related stuff. It contains open source operating sytems and hobby operating systems as one can study their code and learn from them.

• BoneOS- OS for everyone built by everyone
• Cadex OS - a simple operating system kernel originally made by Prof. Douglas Thain at the University of Notre Dame and students
• Clive - A unikernel OS inspired by Plan9 and Nix developed at Universidad Rey Juan Carlos of Madrid
• CollapseOS - Bootstrap post-collapse technology
• FreeRTOS, GitHub - IoT operating system for microcontrollers, by Amazon.
• Genode - A FOSS operating system framework consisting of a microkernel abstraction layer and a collection of userspace components
• HelenOS - multikernel multiserver OS
• Interim - Minimalist OS with concepts from Lisp machines and Plan9
• KnightOS - for z80 calculators
• L4re - Operating system and hypervisor for security/safety-critical and virtualization-enabled applications.
• Minoca OS - General purpose OS, written in C
• NodeOS - OS using NodeJS and Linux
• Redox - written in Rust
• Sculpt OS - Genode based OS
• Thor - 64bit operating system mostly written in C++

• Animal - 32 bit multithreaded operating system (formerly Gramado)
• Cyjon - pure x64 assembly language operating system
• Mako - Hobby OS for x86 from scratch, written in C
• mOS - A hobby operating system developed from scratch
• MyXomycota - Monolithic kernel in C
• nopeos - Simple OS kernel with BASIC interpreter for x86
• Serenity - Graphical Unix-like operating system for x86 computers
• Snowdrop OS - a homebrew operating system from scratch, in assembly language
• Sortix - Hobby OS in C and C++ by Jonas 'Sortie' Termansen
• TempleOS - Religious OS :-)
• ToaruOS - Hobby operating system from scratch
• Týndur - Hobby operating system build by the Lowlevel community. Written in C and Pascal
• Ultron OS - x86 Operating System written in C++, High School Project
• Visopsys - Open Source Hobby OS developed since 1997
• Windows 95 in Electron - Hobby Windows 95 implementation in Electron
• Xv6 - A teaching operating system developed in the summer of 2006 for MIT's operating systems course

• Darwin XNU - The XNU kernel source code for use in MacOS and iOS
• FreeBSD, Github - Unix-like operating system based on the BSD
• Haiku - BeOS inspired OS
• Linux - Linux kernel
• Minix, Github - Unix-like operating system based on a microkernel architecture
• MS-DOS - The original sources of MS-DOS 1.25 and 2.0
• NetBSD, Github - Unix-like operating system based on the BSD
• OpenBSD, Github - Unix-like operating system based on the BSD
• SystemV - AT&T UNIX System V Source Code

• How to Make a Computer Operating System - in C++
• Intermezzos - A follow along book to build IntermezzosOS in Rust
• The little book about OS development by Erik Helin and Adam Renberg
• Think OS - A Brief Introduction to Operating Systems by Allen B. Downey
• Operating System Development Series - OS from the ground up in C
• Operating Systems: Three Easy Pieces - Easy to read book covering virtualization, concurrency and persistence
• xv6: a simple, Unix-like teaching operating system - Book for Xv6
• Operating Systems: From 0 to 1 - Bootstrap yourself to write an OS from scratch by Do Hoang Tu

• Bran's Kernel Development by Brandon Friesen
• How to write a Linux kernel patch and submit it by Greg Kroah-Hartman
• Presentation on how the Linux kernel is developed by Greg Kroah-Hartman
• os-tutorial - How to create an OS from scratch by Carlos Fenollosa
• Writing an OS in Rust by Philipp Oppermann
• Kernels 101 – Let’s write a Kernel by Arjun Sreedharan
• Kernels 201 - Let’s write a Kernel with keyboard and screen support by Arjun Sreedharan

• os-dev - Wiki with everything you need to know
• Lowlevel - A wiki about creating an OS from scratch. In German
• POSIX - Standards for maintaining compatibility between operating systems

• The benefits and costs of writing a POSIX kernel in a high-level language by Cody Cutler, M. Frans Kaashoek, and Robert T. Morris, MIT CSAIL