HaikuOS
Inspired by the BeOS, Haiku is fast, simple to use, easy to learn and yet very powerful.
https://www.haiku-os.org/

RedoxOS
Redox is a Unix-like Operating System written in Rust, aiming to bring the innovations of Rust to a modern microkernel and full set of applications.
https://www.redox-os.org/

SerenityOS
A graphical Unix-like operating system for desktop computers!
SerenityOS is a love letter to '90s user interfaces with a custom Unix-like core. It flatters with sincerity by stealing beautiful ideas from various other systems.
https://serenityos.org/