Rob Pike and the other folks who developed Plan 9 worked at AT&T's Bell Laboratories. Nothing developed in a Bell Lab has ever been 'free' directly. Rather folks who have seen the research and been inspired by it have made changes to their free stuff which you use every day. There are influences from Plan 9 in Linux (the /proc file system for example which was influenced by the SunOS /proc filesystem which was influenced by Plan 9) but that is as far as it goes.