Minix

Minix is a free, educational version of UNIX written by Andrew S. Tanenbaum. It's open source with some restrictions and has a couple of books that accompany it. Tannenbaum created Minix as a small "training" kernel for use in operating system design classes. Because of this educational role, Tannenbaum resisted attempts to extend the Minix kernel, because that would add complexity. Linux started as an attempt by Linus Torvalds to clone Minix, but it obviously evolved beyond that. Tannenbaum was, at one point, involved in a flame war with Linus, complaining about the fact that Linux is a monolithic kernel, rather than a microkernel.