If you are new to Plan 9: don't let the screenshots fool you. They might look like any other late-80s system - but at its heart Plan 9 is much cleaner, more flexible and better thought-out than any other operating system on this earth. Including all of today's Unices.
Plan 9 brought us UTF-8 and tiling window managers. And it invented
/proc, the process file-system now adopted by Linux - to name but a few of Plan 9's innovations. In fact, Plan 9 does everything with file systems - including networking and graphical user interfaces. Things, for which Unix sadly uses sockets and monsters like the X Window System.
Plan 9, on the other hand, is a clean system. A consistent system.
Plan 9 is Unix2.
It just rocks. And it's a great thing to see it alive and kicking. The fork is called Plan 9 from The People's Front of Cat-v.org and lives at google code.
Among other things, they already added USB stick booting, ported mercurial, a Go compiler and Doom, and wrote a bunch of new drivers.
At this rapid pace, it's going to overtake GNU/Linux in... Well, about 10 or 20 years, maybe.
So, jump on the train now, before it's becoming a boring mainstream OS! ;-)
PS: I know, I'm a bit late on this - which is a shame, because I just recently (and before the news) added a "Plan 9 From Bell Labs" Spark (as they call it) on my Google+ profile. I thought that would add new findings about Plan 9 to my stream (like a Google Alert or something). But it seems that I have to navigate to the Sparks page to actually see the news. Which kind of defeats the purpose of adding something as a Spark. I would've seen it, I guess, if someone "shared" it with me, but you know the old joke: two Plan 9 users meet. End-of-joke.