An OS from the folks that gave us UNIX, Plan 9 is a distributed operating system in which everything is a file, unlike UNIX, where most things are files. Based on the RISC approach to operating systems, Plan 9 doesn't have much in the way of features, but the programmer can always create features. It didn't catch on, and the developers went on to work on Inferno.
The excellent Wily editor in UNIX is a clone of Plan 9's Acme editor.