I love using macOS as my personal OS.
MirageOS - Library operating system that constructs unikernels.
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.
Spectrum - Step towards usable secure computing.
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.
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.