Documentation

Startup

QEMU Emulator User Documentation (HTML generated from QEMU sources, updated frequently)
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

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
Use vhost-user with OVS/DPDK as backend

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
Running POWER / sPAPR / pseries guests on QEMU
Running PowerPC Macintosh guests on QEMU
Running SPARC guests on QEMU