Plan 9 from Bell Labs
Plan 9 was originally developed for research purposes, as Bell Labs was looking for a replacement for the venerable UNIX. It underwent mass testing, as all the computers at Bell Labs had Plan 9 installed, in lieu of UNIX, which was commonplace previously. It explored several modifications to the pre-existing UNIX system, primarily the distributed nature of the system, and the graphical user-interface. In 1992, Bell Labs released a public version, for universities, and soon after, a version for the general public. The highly restrictive nature of the licence at the time, and the steep 500$ licencing fee resulted in it being ignored, in lieu of Linux.