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Plan 9
Plan 9 from Bell Labs is a research OS family. Plan 9 was created at Bell Labs by many of the same people who created UNIX, like Ken Thompson, Dennis Ritchie and Rob Pike. It's distinguishing features include the use of files for most, if not all resources in the operating system, and private namespaces, where each process can have a different view of the file system.
Licensing
The First and Second Editions of Plan 9 were only available to universities and then for an exorbitant fee. The Third Edition of Plan 9 was released under the Plan 9 Licence in 2000, which Richard Stallman considered non-free. The Fourth Edition was released under the Lucent Public Licence in 2002 which the FSF does consider free but not GPL-compatible, along with the distribution becoming openly developed and updated daily. In 2014 Plan 9 was relicenced under GPLv2 for UCB's Akaros operating system (to much dislike).
Distributions
Plan 9 from Bell Labs, the Fourth Edition, where development is almost dead.
9legacy, which is several patches on top of the Bell Labs distribution.
9atom is Erik Quanstrom's personal fork of Plan 9. It augments the Plan 9 distribution with the addition of a 386 PAE kernel, an amd64 cpu and terminal kernel, the nupas mail file system, extra PC hardware support, IL and Ken's fs.
Inferno is a distributed operating system started at Bell Labs, but is now developed and maintained by Vita Nuova Holdings as free software.
9front is an active, community-led fork of Plan 9, the life and breath of Plan 9 development, including new advancements such as better hardware support, audio, wireless networking and new programs like NES, SNES, Game Boy, GBA and Genesis emulators and a paint program. If you want to run Plan 9, this is probably the way to go.
