Minix is a Unix clone written by Andy Tanenbaum, along with several of his students and a host of other contributors. In an anecdote, Andy explains why Minix was written:
I was teaching a course on operating systems and using John Lions' book on UNIX Version 6. When AT&T decided to forbid the teaching of the UNIX internals, I decided to write my own version of UNIX, free of all AT&T code and restrictions, so I could teach from it. My inspiration was not my time at Bell Labs, although the knowledge that one person could write a UNIX-like operating system (Ken Thompson wrote UNICS on a PDP-7) told me it could be done. My real inspiration was an off-hand remark by Butler Lampson in an operating systems course I took from him when I was a Ph.D. student at Berkeley. Lampson had just finished describing the pioneering CTSS operating system and said, in his inimitable way: "Is there anybody here who couldn't write CTSS in a month?" Nobody raised his hand. I concluded that you'd have to be real dumb not to be able to write an operating system in a month. The paper cited above is an operating system I wrote at Berkeley with the help of Bill Benson. It took a lot more than a month, but I am not as smart as Butler. Nobody is.
I set out to write a minimal UNIX clone, MINIX, and did it alone. The code was 100% free of AT&T's intellectual property. The full source code was published in 1987 as the appendix to a book, Operating Systems: Design and Implementation, which later went into a second edition co-authored with Al Woodhull. MINIX 2.0 was even POSIX-conformant. Both editions contained hundreds of pages of text describing the code in great detail.
Within a couple of months of its release, MINIX became something of a cult item, with its own USENET newsgroup, comp.os.minix, with 40,000 subscribers. Many people added new utility programs and improved the kernel in numerous ways, but the original kernel was just the work of one person: me.