Documentation (as of Sun Feb 24 03:34:08 CET 2013)

Using rio - An introduction to using the window system.
Using acme - Acme is a hybrid editor and shell/windowing application.
Using plumbing - The plumber sends messages to the appropriate program.
9p services using srv, listen, exportfs, import - How Plan 9 systems communicate.
Fonts - for the screen and otherwise.
Using troff - Formatting text for printing on a typesetter.
Using vncv - Remote use of applications on other OSes.
Using SSL - How to store keys and certificates.
Using aescbc to store factotum keys - secstore(1) for stand-alone terminals.
Connecting to other OSes - ssh, telnet, u9fs, SMB, NFS...
Connecting from other OSes - drawterm, v9fs, 9vx...
X11 installation - Functional but not recommended.
Linux emulation - Rudimentary support for running Linux programs on Plan 9.
UNIX to Plan 9 command translation.
Tip o' the day

Drawterm to your terminal
Console messages
Using PQ - The implicit relational database client and server.
Configuring file-backed Venti stores
Venti-Rescue - How to rebuild your system when your disks crash.
Kfs file system configuration - Of historical interest; kfs is now deprecated.
Installing a Plan 9 File Server - Instructions for installing the old dedicated file server; in most cases it is recommended that you use fossil instead.
Mouse vs. keyboard
Dialup modem config
Installing Plan B on Qemu

Documentation