Plan 9 from Bell Labs started as a distributed operating system, intended to succeed Unix at Bell Labs, in the mid 80's. Since then, there have been four Editions published, each including all the source code, and the most recent of which is Open Source (and Free Software, take your pick). Over time Plan 9 has attracted the interest and contributions of a growing community of researchers and enthusiasts from around the world.
Plan 9 itself has spawned several related systems, notably Plan 9 from User Space (Plan 9 Ports), which is a version of most of the Plan 9 user-level environment that runs under Unix, and Inferno, an all-in-one distributed system with kernels closely related to Plan 9 but adding a virtual machine and concurrent programming language.
There are projects here in each of those areas. Several projects have components in all of them.