KA9Q Mirror
An impressive early implementation of TCP/IP
...KA9Q NOS attracted many contributors and became widely used throughout the 1980s and 1990s in amateur packet radio and various educational projects. In a way, it was the Linux of its day.
KA9Q NOS became the basis for several low-end commercial dialup terminal servers and routers. It influenced the development of the Internet protocols and implementations, including the Linux kernel.