BSD Berkeley Software Distribution
The Berkeley branch of Unix that gave the world sockets, the TCP/IP stack, and a licence permitting almost anything.
Developed at the University of California from 1977. The BSD stack became the TCP/IP implementation and was copied into Windows, macOS and countless embedded systems. vi, csh and the sockets API all come from here.
The USL lawsuit of 1992–1994 over copyright froze the project for two critical years — precisely as Linux was taking off, which many argue decided which free Unix would dominate. It survives in FreeBSD, OpenBSD and NetBSD, and in the macOS userland.