Plan 9 is a fully-featured, clean, compact and uniform operating system built as a successor to Unix.
Plan 9 is not a product, it is an experimental investigation into a different way of computing. The developers started from several basic assumptions: that CPUs are very cheap but that we don't really know how to combine them effectively; that good networking is very important; that an intelligent user interface is a Right Decision; that existing systems are not the correct way to do things, and in particular that today's workstations are not the way to go.
Relevance to permacomputing
The Plan 9 system is likely one of the fastest and most complete operating system for the Raspberry Pi devices. In comparison with the various compatible Linux and BSD distribution, it comes already bundled with all the software and documentation required to understand and expand its inner workings.
"An argument for simplicity and clarity."
— Rob Pike