(Some services like audio don't die easily.) Modules I unload: The whole audio stack, usb_storage, webcam drivers, wireless, bluetooth. Services I usually turn off include: Ubuntu One, ssh, apache, databases, avahi, pulseaudio, cups, apparmor, acpi-daemon, bluetooth.
Look at output from ps aux to see what's running, and kill -9 to terminate them. Sometimes you also need to kill daemons or programs that are using the service or driver before they can be turned off. If/when you want to bring things back, easiest way is to simply reboot. Use lsmod to see what kernel modules are loaded, and rmmod to unload it. Use service -status-all to see what's running on your system, and service stop to shut it down. I've been able to more than double the battery life of my netbook by turning off a bunch of services I don't need, and unloading their kernel drivers.
