Documentation

Startup

QEMU crash course
QEMU documentation on wikibooks.org
QEMU-Buch / QEMU-Book - a quite versatile book on qemu, provided in German and English (partly as a google translation), its describing e.g. lots of guests and hosts

for users

Set up Networking
Setting up VirtFS (9P Sharing over Virtio) between the guest and host
Create a Snapshot
Ubuntu Development Ports - details on qemu-<arch> usage, chroot set up, system emulation and cross compilation
Compiling with qemu-user chroot - details on tuning binfmt_misc with magic patterns for making alien binaries run from the shell with silent invocation of QEMU

for users that target for a specific platform

Debian on an emulated ARM machine based upon some Debian Linux host (see here for the probably most current images matching this HowTo)
Debian on an emulated MIPS(EL) machine based upon some Debian Linux host
Debian on an emulated SPARC machine based upon some Debian Linux host
Windows XP on QEMU-x86 based upon some Linux host (brief but good hints on the guest set up)
FreeDos on QEMU-x86 based upon some Linux host (concentrates on host details)
QEMU in Embedded Linux Development - Virtual Development Board

for developers

QEMU Internals (HTML generated from QEMU sources, updated frequently)
QEMU, a Fast and Portable Dynamic Translator, USENIX 2005 Annual Technical Conference, FREENIX Track (NB: dyngen is obsolete and has been replaced by TCG)
The QCOW Image Format
SEO
Hardware Manuals
Instruction Set Manuals (Assembly Language)
PC Platform