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.
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: