I wrote a little more about Plan 9 in my additions to the go9p library:
9P is the network protocol used by the mythical Plan9 operating system. The big idea is that all resources, networked or local, can be represented as read/write operations on named hierarchies of objects. Accessing those should be just as easy as accessing a file, and it shouldn't make any difference if the file represents a local or remote resource. For modern re-inventions of this, see REST API design and FUSE. Although revolutionary in its scope, the OS was not initially released open source and required some effort in porting existing software, hindering widespread adoption. Its relatively small following led Eric S. Raymond, one of its more vocal designers, to lament that "the most dangerous enemy of a better solution is an existing codebase that is just good enough."
After the decision to leave the personal computer and Unix networked server markets in 1996, AT&T divested the National Cash Register Corporation and spun-off Bell Labs as Lucent Technologies, Lucent gave Plan9 a back-seat and development slowly declined. In 2002, Lucent made its last (4th edition) release of the Plan9 OS. Interestingly, this nearly coincides with the date that Rob Pike moved from Lucent to Google. Its major issue is the lack of support for most hardware. It spewed some random garbage and ground to a halt when I tried to boot it on my AMD 2700-based system.
In 2011, a fork (9front) of the slowly developing v4 was made to allow faster development, addition of new hardware support, and a re-write of the fossil filesystem to the "Cached Write Once Read Many Filesystem." All I know is that this one did boot on my (ca. 2002) hardware.