A kernel is the heart of almost every operating system. It is always loaded in memory at any time and deals with the hardware to provide an interface for the software. It also manages peripherals, memory, interrupts, and processes. Examples of widely used kernels include Windows NT and Linux.
RT-Thread is an open source IoT Real-Time Operating System (RTOS).
A proof-of-concept microkernel that implements a WebAssembly "usermode" that runs in Ring 0.
Pragmatic microkernel-based OS with fully asynchronous I/O
A portable microkernel-based multiserver operating system written from scratch.
Genode OS Framework
华为鸿蒙分布式操作系统（Huawei HarmonyOS，纯血鸿蒙Harmony Next ），开源鸿蒙分布式操作系统（ OpenHarmony）开发技术交流，最全鸿蒙技术资料库，手册，指南，共建国产操作系统万物互联新生态。
🏢 An operating system inspired by brutalist design that combines the ideals of UNIX from the 1970s with modern technology and engineering
MIT 6.828 Operating System Lab https://pdos.csail.mit.edu/6.828/2018/schedule.html
FreeNOS (Free Niek's Operating System) is an experimental microkernel based operating system for learning purposes written in C++. You may use the code as you wish under the terms of the GPLv3.
An efficient and secure microkernel built for ARM Cortex-M cores, inspired by L4
Ghost, a micro-kernel based hobby operating system.
NOVA Microhypervisor
Experimental WASM Microkernel Operating System
A hobbyist microkernel written in Rust, featuring a capability-based system similar to seL4.
Microkernel and userspace written in Rust exploring modern ideas