[vezzy-fnord]

Linux will never even be anything close to Plan 9 for the simple reason that the Research Unix model is incompatible with that of Plan 9. It's become fashionable among certain segments of Linux userspace developers to deny the fundamental grounding Linux has in Unix and try to go "beyond the antiquated Unix model"... into OS X, another Unix. There's no other way it could be, really.

That said, the Linux kernel developers have had some ideas trickle down. They use virtual file systems quite extensively for both internal and external interfaces. Without per-process namespaces, their take on procfs simply isn't the same, but it is an approximation. A proper mechanism for union mounts called overlayfs was merged recently, after a rather protracted ordeal.

The kernel has also had a 9P client called v9fs for a while now, but I'm not sure how much it's maintained these days.

On the other hand, there's nothing like an auth server similar to Factotum (which deprecates superuser) because it contradicts the Unix permission model, there is nothing like plumber(4) inter-application communication system (D-Bus is a completely different thing altogether), the Plan 9 approach to cross-compilation isn't present (though this is not specifically a Linux issue at all), it's still conventional to use POSIX shells instead of cleaner designs like rc(1) and so forth.

The Fossil file system and Venti archival file server are also pretty unique to Plan 9, though their general functions have been emulated in more complicated manners by the myriad of Linux file systems.