It's far from comprehensive, but here are a handful of operating system projects that have caught my eye:

[1] MezzanoOS, an OS written in Common Lisp (https://github.com/froggey/Mezzano)

[2] SerenityOS, a Unix-like OS (https://github.com/SerenityOS/serenity/). The primary author has a youtube channel (https://www.youtube.com/c/AndreasKling/)

[3] RedoxOS, a microkernel OS written in Rust (https://www.redox-os.org/)

[4] Nebulet, a microkernel based on webassembly (https://github.com/nebulet/nebulet)

[5] It was never released by Microsoft, but Midori OS had some fascinating ideas like software isolated processes, asynchronous message passing and object capabilities. Nice write-up about the project by Joe Duffy here http://joeduffyblog.com/2015/11/03/blogging-about-midori/