PureDarwin
Moving the Darwin community in the right direction!
Independent OS
9front System
AmigaOS
Arca Noae
Arca Noae is an OS/2-focused software and development company, providing expert consulting for a wide variety of OS/2 and eComStation installations.
AROS
FreeDOS
FreeDOS is an open source DOS-compatible operating system that you can use to play classic DOS games, run legacy business software, or develop embedded systems.
Friend OS
Friend gives you control of your own data, on your own online computer on locally managed servers.
Haiku OS
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.
Minix3
MorphOS
openMSX
RISC OS
ROS.org
TempleOS