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
- 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