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.
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.