Plan9 is an OS and a set of userspace tools. They had some rather nice design ideas, like making everything a file, and let users interact with that.
The main problem is that Plan9 don’t really run on any usefull hardware – it’s kinda limited in it HW support. To overcome that, Glendix is making a Plan9/Linux – a Linux kernel with Plan9 userspace tools…
We’ve seen GNU/Solaris, GNU/HP-UX, GNU/BSD and, most commonly, GNU/Linux. However, we seldomly see */Linux. It almost goes without saying that Linux uses GNU user-space tools. One outcome of Plan9/Linux might be that RMS finally gets GNU/Linux more prominent… However, I’m not holding my breath for that 😉