The University of California, Berkeley, has been authorised by Alcatel-Lucent to open sauce all Plan 9 software under the GNU General Public License, Version 2.
This means that you can redistribute it and/or modify it if you reall want.
In a press release, the University said that it was being distributed without any warranty in the hope that it will be useful.
Plan 9 (from User space, geddit?) was a Bell Labs internal project which was designed to replace Unix as Bell Labs's primary platform for operating systems research.
It was designed to handle multi-user environments. It was released in 1992. In the late 1990s, Lucent Technologies, having inherited Bell Labs, dropped commercial support for the project. In 2000, a non-commercial third release was distributed under an open source license. A fourth release under a newfree software license occurred in 2002.
Plan 9 has never approached Unix in popularity, mostly because it was not much of an improvement on Unix.