OpenSSH is most prominent implementation of the SSH protocol. I can’t imagine my life without OpenSSH. Almost all of my devices / server / network equipment such as routers and tiny embedded device has OpenSSH these days.
From OpenBSD journal:
Eight years ago today, Sept 26 1999, Theo de Raadt committed the initial source code for OpenSSH to the OpenBSD repository. The code was a fork of Björn Grönvall's OSSH, which was derived from an early version of the increasingly "less free" ssh from Tatu Ylönen.