Plan 9 is open sauced
 

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.

http://news.techeye.net/software/plan-9-is-open-sauced