"We all know what Linux is, but what exactly is Plan 9? Plan 9,
developed at Bell Labs, was supposed to become the successor to
UNIX, also developed at Bell Labs. Plan 9 was written from scratch,
and made many radical departures from standard UNIX conventions.
For instance, in Plan 9, everything really is a file; even the
window a program is running in is represented as a file in a
hierarchical file system. Every program in Plan 9 sees every
possible resource as a file. Plan 9 is also fully distributed, so
that parts of the operating system can run on different machines.
Plan 9 never made its way out of the research departments, and the
reason is fairly simple: UNIX is good enough. It might not be
exactly elegant, but it gets the job done."