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.
Plan 9’s latest release is it’s fourth edition. This site is a mirror for the Plan 9 ISO, the Plan 9 Raspberry Pi image and sources.
Here are some more places where you can find software for Plan 9, although they are not mirrored (they may be in the future):
And a few other links that may be of interest: