MINIX Alive
Tanenbaum's teaching microkernel — Linux was built in its newsgroup, and a version hides inside every Intel CPU's Management Engine.
Andy Tanenbaum wrote MINIX in 1987 as a teaching OS — 12,000 lines, microkernel, POSIX-ish, source in the book. It was the first microkernel OS shipped to students.\n\nTorvalds used it as a scaffold while writing Linux; Tanenbaum's 1992 'Linux is obsolete' Usenet argument with him is the most famous flamewar in OS history. MINIX 3 (2005) went BSD-licensed; it's the microkernel inside Intel's Management Engine — meaning it's the most-deployed OS on Earth.