Probably the most important one to know is the "reboot" combo for when the iPod Touch crashes. Er, I mean if. Most of the rest of these were discovered while trying to reboot my crashed iPod Touch.
There are basically two buttons on the iPod Touch: the Sleep/Wake button on the top, and the Home button on the face. Rather than write "Sleep/Wake" out each time, I'm just going to call it the Sleep button.
So, here are the button combos:
Hold Sleep for 5 seconds: bring up the "turn off iPod" UI, or turn the iPod back on if it's off.