I love using macOS as my personal OS.
Toddler - Well-designed usable and portable microkernel OS.
OS67 - Unix-like toy kernel.
Resea - Pure microkernel-based hackable operating system written from scratch in C and Rust.
Rux - Hobbyist microkernel written in Rust, featuring a capability-based system similar to seL4.
redshirt - Experiment to build some kind of operating-system-like environment where executables are all in WASM and are loaded from an IPFS-like decentralized network.
mikado - RISC-V Experimental OS.
Barebones - Barebones multiboot kernel for beginners.
Orange Slice - Research kernel and hypervisor attempting to get fully deterministic emulation with minimum performance cost.
Cloud Hypervisor - Open source Virtual Machine Monitor (VMM) that runs on top of KVM.
RancherOS - Tiny Linux distro that runs the entire OS as Docker containers.
Bottlerocket OS - Free and open-source Linux-based operating system meant for hosting containers.
XNU kernel - Part of the Darwin operating system for use in macOS and iOS operating systems.
Pebble - Microkernel and userspace written in Rust, with first-class message passing.
RustyHermit - Rust-based, lightweight unikernel.
Distaff - Zero-knowledge virtual machine written in Rust.
zCore - Reimplement Zircon microkernel in Rust.
lilith - POSIX-like x86-64 kernel and userspace written in Crystal.
Genode - Operating System Framework.
Shrine - TempleOS distro for heretics.
LegoOS - Disseminated, Distributed OS for Hardware Resource Disaggregation.
CuBitOS - General-purpose, formally-verified, 64-bit operating system in SPARK/Ada for x86-64.
Demikernel - Library operating system architecture designed for use with kernel-bypass I/O devices.
CyanOS - x86 monolithic kernel operating system, coded in C++ 17 and few lines of x86 Assembly.
ChaiOS - Modular multi-platform hobby OS. Hybrid kernel, largely monolithic. Kernel C library is dynamically linked.
HalideOS - Experimental operating system written entirely from scratch. It supports a basic shell with some commands including a command line calculator.
Managarm - Pragmatic microkernel-based OS with fully asynchronous I/O. (Fully Asynchronous OS Based on Modern C++ - Alexander Grinten (2020))
KnightOS - Third-party operating system for TI calculators.
Stupid Operating System - Simple, tiny toy OS implemented in Rust.
CactusOS - Simple operating system that is aimed to not be anything like linux.
MollenOS - Modern operating system that is built with focus on abstraction and a modular design.
Azure RTOS ThreadX - Advanced real-time operating system (RTOS) is designed specifically for deeply embedded applications.
bootloader - Experimental x86 bootloader written in Rust and inline assembly.
Cyjon - Simple, clean, multi-tasking operating system written in pure assembly language for 64-bit processors from the AMD64 family.
Contiki - Open source operating system that runs on tiny low-power microcontrollers.
Rumprun - Uses rump kernels to provide the Rumprun unikernel. Rumprun works on not only on hypervisors such as KVM and Xen, but also on bare metal.
Rump Kernels - Enable you to build the software stack you need without forcing you to reinvent the wheels.
mOS - Unix-like operating system developed from scratch and aims to POSIX compliant.
pdp7-unix - Project to resurrect Unix on the PDP-7 from a scan of the original assembly code.
core-os-riscv - xv6-like operating system on RISC-V with multi-core support.
rCore - Rust version of THU uCore OS. Linux compatible.
Eltanin - Free, simple, and complete general-purpose operating system.
Pluto - x86 kernel written in Zig.
Mimosa - Operating system consists of a minimal kernel built on C++ and Scheme.
daintree - ARMv8-A operating system, plus a UEFI bootloader, all written in Zig.
M3 - Microkernel-based system for heterogeneous manycores.
vanadinite - RISC-V OS written in Rust.
Helium - BSD-based OS project that aims to provide an experience like and some compatibility with macOS.
TypOS - Operating system for typechecking processes.
Kerla - New operating system kernel with Linux binary compatibility written in Rust.
rustboot - Tiny 32 bit kernel written in Rust.
mycelium - Hobby OS in Rust.
FuzixOS - Because Small Is Beautiful.
We have moved away from a world of shared libraries, filesystems, and UNIX users and permissions into a world of shared-nothing (no shared memory, no shared filesystem), capabilities, new and extremely aggressive attack vectors, and a need to compartmentalize and virtualize at more fundamental layers even if it comes at a performance penalty. You can't retrofit a microkernel-like abstraction on top of Linux. At the same time, a lot of the features you need for a shared multi-user system are basically cruft for modern mobile, single-user systems with little use for shared resources (not saying they're not shared; it's just that you can no longer trust apps installed in the user system so expecting apps to behave nicely is out of the window). The new wave of OSes embraces formal correctness when possible, JIT, garbage-collected application programming languages, tightly-enforced resource boundaries, deny-by-default security models, provably-safe system programming languages (Rust and whatever else will come), immutability and copy-on-write at the cost of filesystem space, and secure memory abstractions for more RAM.
mirageOS - A programming framework for building type-safe, modular systems.
Raspberry Pi OS - Learning operating system development using Linux kernel and Raspberry Pi.
Plan 9 from User Space - Port of many Plan 9 libraries and programs to Unix.
Unik - Unikernel & MicroVM Compilation and Deployment Platform.
SPDK - Storage Performance Development Kit Provides a set of tools and libraries for writing high performance, scalable, user-mode storage applications.
intermezzOS kernel - Hobby operating system, in Rust.
Nanos - New kernel designed to run one and only one application in a virtualized environment.
Soso - Simple unix-like operating system written in Nasm assembly and mostly in C.
swieros - Tiny and fast Unix-ish kernel (based on xv6), compiler, and userland for fun, education, and research.
ioping - Simple disk I/0 latency measuring tool.
Awesome eBPF - BPF, as in Berkeley Packet Filter, is an in-kernel virtual machine running programs passed from user space.
Krabs - x86 bootloader written in Rust.
oreboot - Fork of coreboot, with C removed, written in Rust.
fectl - Client/server system that allows its users to monitor and control a number of processes on UNIX-like operating systems.
HiddenVM - Use any desktop OS without leaving a trace.
Solo5 - Sandboxed execution environment for unikernels.
Albatross - Orchestrate and manage MirageOS unikernels with Solo5.
Zub VM - Super-fast, stack-based virtual machine for dynamic languages.
Modern Operating Systems 3rd edition book - 4th edition available too.
[Operating Systems Design & Implementation MINIX Book](http://index-of.es/EBooks/Operating%20Systems%20Design%20&%20Implementation%203rd%20Edition(1).pdf)
SBTCVM (Simple Balanced Ternary Computer Virtual Machine) - Project focused on providing virtual machines based around balanced ternary computers.
Tarides - Building functional systems. Commercial support on MirageOS and Irmin.
Systems with JT - YouTube - YouTube channel doing first impression reviews of amateur operating systems.
RVM 1.5 - Type-1.5 hypervisor written in Rust.
OpenRC - Dependency-based init system that works with the system-provided init program, normally /sbin/init.
Awesome Virtualization - Curated list of awesome resources about virtualization.
pyvmidbg - LibVMI-based GDB server, implemented in Python.
libmicrovmi - Cross-platform unified interface on top of hypervisor's VMI APIs.
crosvm - Chrome OS Virtual Machine Monitor.
Rust VM - Virtual Machine written in Rust to emulate the LC3 computer (Little Computer 3).
Microkernel.info - List of active free, open source microkernel projects.
Sudo - Program designed to allow a sysadmin to give limited root privileges to users and log root activity.
uhyve - Minimal hypervisor for RustyHermit.