skiftOS
skiftOS is a hobby operating system built for learning and for fun targeting the x86 platform. It features a kernel named hjert, a graphical user interface with a compositing window manager, and familiar UNIX utilities.
OS Other
urbit.org
QEMU
A generic and open source machine emulator and virtualizer
Your favorite operating systems in one place! | netboot.xyz
netboot.xyz enables you to PXE boot many Operating System installers and utilities from a simple to use menu powered by the iPXE project.
Haiku Project
Haiku is an open-source operating system that specifically targets personal computing. Inspired by BeOS, Haiku is fast and easy to learn but very powerful.