1983: GNU - History of Desktop OSs
GNU is developed as a UNIX-like operating system by the GNU Project, led by Richard Stallman, founder of the Free Software Foundation. GNU is designed to be a "complete Unix-compatible software system" built with entirely free software. Following the announcement in 1983, the actual development began in 1984.
A first test release of a complete GNU system was published in 1996, based on the GNU Hurd kernel that was developed since 1990. Even today, the GNU Project says that Hurd "still lacks some important features, so it is not widely used". So far there has not been a complete production release, but "official GNU software packages" are offered instead.