Plan 9 from Bell Labs
Current Work
The operating system is still being used and developed by a small group of people, the mailing list 9fans is pretty active.
There are ports from parts of it for Unix-like systems, see Plan 9 from User Space.
Design principles such as the file system are inspiring new software, see the group suckless.org and its window manager wmii.
Rob Pike and Ken Thompson are now at Google creating the programming language Go, see golang.org.
Plan 9 Official Site
Other Sites
NineTimes — Plan 9, Inferno, Unix and Bell Labs operating systems news
Documents
Newbie guide 
by Michael Covington (15 pages, 2008).
Book about Plan9 in German on Lulu.com (237 pages, 2006).
Introductory OS-Course by Francisco J. Ballesteros on Lulu.com (385 pages, 2009). There is also a draft (2007) available.
Installation
The installation on a QEMU Virtual Machine on GNU/Linux (Debian) is quite straight forward.
I adapt here the instructions found for Fedora at this blog post (German). See Installing Plan 9 on Qemu on the Plan 9 wiki, too.
1. Download ISO CD-ROM Image
wget http://plan9.bell-labs.com/plan9/download/plan9.iso.bz2
bunzip2 plan9.iso.bz22. Install QEMU-KVM Virtual Machine
apt-get install kvmCreate a 1 GB blank disk for the virtual machine.
dd if=/dev/zero of=plan9.dsk bs=1024 count=10485763. Install
kvm -cdrom plan9.iso plan9.dsk -m 512Follow the installation instructions.
4. Run
kvm plan9.dsk -m 512 &Log in as "glenda".
