I love using macOS as my personal OS.
Toddler - Well-designed usable and portable microkernel OS.
OS67 - Unix-like toy kernel.
Serenity - X86 Unix-like operating system for IBM PC-compatibles.
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.
Mezzano - Operating system written in Common Lisp.
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))
EggOS - Go unikernel running on x86 bare metal.
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.
Jehanne - Simple operating system.
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.
Operating Systems: From 0 to 1 - Bootstrap yourself to write an OS from scratch. A book for self-learner.
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.