Free Software Foundation
The FSF is a charity with a worldwide mission to advance software freedom.
FOSS, Software

The GNU Operating System and the Free Software Movement
Since 1983, developing the free Unix style operating system GNU, so that computer users can have the freedom to share and improve the software they use.
GNU, FOSS, Software

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