Debian -- The Universal Operating System
Debian is an operating system and a distribution of Free Software. It is maintained and updated through the work of many users who volunteer their time and effort.
FOSS, Software, Operating Systems

Haiku
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.
FOSS, Software, Operating Systems

KolibriOS
KolibriOS is a lightweight and powerful OS that runs on a few megabytes of disk space and 8MB of RAM. It has a rich set of applications, supports various file systems and drivers, and boots in seconds.
FOSS, Software, Operating Systems

ReactOS
Imagine running your favorite Windows applications and drivers in an open-source environment you can trust. That's the mission of ReactOS!
FOSS, Software, Operating Systems