The Linux and BSD /proc is a pale imitation of plan9's, though. The most interesting ideas in plan9 are inherently incompatible with Unix because they're direct consequences of a deep overhaul of its security and file/namespace systems. (Much deeper than, say, adding sudo.) The free Unices' /proc filesystems attach to the global filesystem namespace, but on plan9 each process can have its own, and consequently what you can do with them is very different.

(FWIW, I detest the plan9 GUI, and am more interested in microkernel-based approaches such as in Minix 3, but getting into that in any real way is so far down my spare-time-fun list that I can only consider myself a curious onlooker.)