Plan 9 from Bell Labs
Plan 9 is a research system developed at the Bell Labs. Designed to be the successor for the UNIX operating system, it makes up for many of the mistakes that UNIX made - offering a completely new way of looking at operating and distributed systems. Plan 9 was written from scratch, and emphasizes on high code quality with minimal complexity.
The latest available Plan 9 ISO image is from November 21, 2007
Here are some more places where you can find software for Plan 9, although they are not mirrored (they may be in the future):
