Minix (Mini Unix)

MINIX is a Unix-like computer operating system based on a microkernel architecture created by Andrew S. Tanenbaum for educational purposes; MINIX also inspired the creation of the Linux kernel.
MINIX (from "mini-Unix") was first released in 1987, with its complete source code made available to universities for study in courses and research. It has been free and open source software since it was re-licensed under the BSD license in April 2000.

At the time of its original development, the license for MINIX was considered to be rather liberal. Its licensing fee was very small ($69) compared to those of other operating systems. Although Tanenbaum wished for MINIX to be as accessible as possible to students, his publisher was not prepared to offer material (such as the source code) that could be copied freely, so a restrictive license requiring a nominal fee (included in the price of Tanenbaum's book) was applied as a compromise. This prevented the use of MINIX as the basis for a freely distributed software system.
When free and open source Unix-like operating systems such as Linux and 386BSD became available in the early 1990s many volunteer software developers abandoned MINIX in favor of these. In April 2000, MINIX became free/open source software under a permissive free software licence, but by this time other operating systems had surpassed its capabilities, and it remained primarily an operating system for students and hobbyists.