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.
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.