• AROS - Research Operating System is a lightweight, efficient, and flexible desktop operating system, designed to help you make the most of your computer. It's an independent, portable and free project, aiming at being compatible with AmigaOS at the API level
• Clive - A unikernel OS inspired by Plan9 and Nix developed at *Universidad Rey Juan Carlos of Madrid*
• dahliaOS - dahliaOS is a modern, secure, lightweight and responsive operating system, combining the best of GNU/Linux and Fuchsia OS.
• Embox - Configurable operating system kernel designed for resource constrained and embedded systems.
• ExectOS - sys/exectos) - ExectOS is a modern, EFI-enabled, general purpose operating system written from scratch and implementing the XT architecture. It runs on x86 and x86_64 architectures and provides NT drivers compatibility layer.
• Fiwix - A UNIX-like kernel for the i386 architecture.
• FreeRTOS - 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
• GreenteaOS - A free OS designed to be compatible with Windows executables
• HarveyOS - A distributed operating system
• Hubris - Hubris is an operating system for microcontrollers developed by Oxide Computer Company in Rust
• Illumos - illumos is a Unix operating system which provides next-generation features for downstream distributions (like OmniOS, OpenIndiana, and Tribblix)
• Jehanne - OS inspired by Plan9, 9front and Harvey OS
• Mimiker - Simple unix-like operating system for education and research purposes. MIPS microkernel.
• NESOS - An OS for the Nintendo Entertainment System!
• Sanos - Sanos is a minimalistic 32-bit x86 operating system kernel for network server appliances running on standard PC hardware.
• skiftOS - A simple, handmade, operating system with a graphical user interface.
• Sculpt OS - Genode based OS
• tock - A secure embedded operating system for microcontrollers
• LemonOS - OS) - UNIX-like 64-bit operating system written in C++.
• Snowdrop OS - a homebrew operating system from scratch, in assembly language
• Sortix - Hobby OS in C and C++ by Jonas 'Sortie' Termansen
• Syllable - Successor of AtheOS, Syllable is designed in the tradition of the Amiga and BeOS, but built using many parts from the GNU project and Linux.
• tock - A secure embedded operating system for microcontrollers
• BareMetal - 64 bit operating system written in Assembly for x86-64
• MenuetOS - Hobby OS supporting 32 and 64 bit, written in Assembly language
• MyXomycota - Monolithic kernel in C
• PathOS - Hobby OS based on [MikeOS](https://github.com/mig-hub/mikeOS), written in Assembly Language
• PonyOS - ToaruOS-based, My Little Pony themed OS
• PureDarwin - PureDarwin is a community project to extend Darwin into a complete, usable operating system
• Visopsys - Open Source Hobby OS developed since 1997
• Xv6 - A teaching operating system developed in the summer of 2006 for MIT's operating systems course
• KolibriOS - MenuetOS fork
• 9front - A fork of Plan 9, designed for distributed, networked computing.
• AlmeidaOS - x86-64 OS with its own bootloader, scheduler and limited libc all written from scratch
• Animal - 32 bit multithreaded operating system (formerly Gramado)
• AurixOS - A small multiplatform Operating System designed for developers and creators
• Basekernel - A simple OS kernel for research, teaching, and fun
• BoneOS - OS for everyone built by everyone
• Bottlerocket OS - Linux-based OS meant for hosting containers.
• Brutal - An operating system inspired by brutalist design that combines the ideals of UNIX from the 1970s with modern technology and engineering
• Charlotte - CharlotteOS kernel in Rust
• Cyjon - pure x64 assembly language operating system (related to [Fern-Night](https://github.com/CorruptedByCPU/Fern-Night/) project).
• Dreamos64 - An x86-64 hobby os built from scratch
• Fern-Night - C language operating system (related to [Cyjon](https://github.com/CorruptedByCPU/Cyjon/) project).
• Fomos - Experimental OS, built with Rust.
• GarnOS - Hobby OS powered by a modular kernel.
• Gloire - An OS built with the [Ironclad](https://www.nongnu.org/ironclad/) kernel (a partially formally verified kernel with a small footprint for general-purpose and embedded OSes, written in SPARK and Ada) and GNU tools.
• HalideOS - experimental operating system written entirely from scratch.
• HelenOS - multikernel multiserver OS
• Interim - Minimalist OS with concepts from Lisp machines and Plan9
• JSD-OS - A small operating system for 32 bit x86.
• Animal - 32 bit multithreaded operating system (formerly Gramado)

Hobby

• AquilaOS - Operating System that is designed to be POSIX compliant and mostly ISA transparent
• House - Haskell User's Operating System and Environment.
• KolibriOS - MenuetOS fork
• Týndur - Hobby operating system build by the Lowlevel community. Written in C and Pascal

Popular Operating Systems

• FreeDOS - DOS compatible OS
• Minix - MINIX-Research-Foundation/minix) - Unix-like operating system based on a microkernel architecture
• NetBSD - Unix-like operating system based on the BSD
• OpenBSD - Unix-like operating system based on the BSD
• Plan 9 from Bell Labs - An OS from the creators of Unix, extending the Unix philosophy of "everything is a file" with a network-centric file system, namespaces and distributed computing.
• ReactOS - A free Windows-compatible Operating System
• SystemV - AT&T UNIX System V Source Code
• Plan 9 from Bell Labs - An OS from the creators of Unix, extending the Unix philosophy of "everything is a file" with a network-centric file system, namespaces and distributed computing.

Online books

• Linux Kernel in a Nutshell - Covers the entire range of kernel tasks, available as chapters or one PDF
• The little book about OS development
• 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: Three Easy Pieces - Easy to read book covering virtualization, concurrency and persistence

Tutorials

• Baking Pi – Operating Systems Development
• Bran's Kernel Development
• Roll your own toy UNIX-clone OS
• Writing an OS in Rust
• Making a RISC-V Operating System using Rust
• Operating systems development for Dummies
• Kernels 101 – Let’s write a Kernel
• Kernels 201 - Let’s write a Kernel with keyboard and screen support
• Writing a Tiny x86 Bootloader
• Writing a Bootloader

Websites

• Genodians - Stories around the Genode Operating System
• Lowlevel - A wiki about creating an OS from scratch. In German
• POSIX - Standards for maintaining compatibility between operating systems
• Rust OSDev - Newsletter about OS development in Rust
• os-dev - Wiki with everything you need to know
• OSRTOS - List of open source real-time operating systems
• Multicians - The Multicians web site presents the story of the Multics operating system for people interested in the system's history

Website

• os-dev - Wiki with everything you need to know

Papers

• The benefits and costs of writing a POSIX kernel in a high-level language
• Running Scheme On Bare Metal

Video Tutorials

• Write your own Operating System
• Nanobyte
• Kernel dev from scratch by Dragon Zap Education
• OSDev Lecture Series by AptRock327

Linux Kernel specific Resources

• Linux Kernel Teaching
• OldLinux - A Heavily Commented Linux kernel source and more
• Linux kernel system call table for all archs
• A Beginner’s Guide to Linux Kernel Development (LFD103)

Video Tutorials

• The Linux Kernel documentation
• Working with the kernel development community
• linux-insides
• Linux Kernel Workbook
• Linux Kernel Teaching
• bootlins interactive Linux kernel map
• OldLinux - A Heavily Commented Linux kernel source and more
• syzbot

Blogposts

• You can be a kernel hacker!
• 4 paths to being a kernel hacker
• Guessing Linux kernel registers
• Planet Kernel
• Fixing bugs in the Linux kernel with Syzbot, Qemu and GDB
• netdev in 2023
• Five commands to crash the kernel