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.
Double-click Home: Pop up a dialog that allows basic iPod controls (play/pause, skip, that sort of thing). This is also works when the iPod is locked. (This may be iPod Touch only.)
Press and hold Home for six seconds: force-quit the current application.
Press Sleep and Home: take a screenshot. (Screenshots are stored as photos, which I assume makes more sense with the iPhone.)
Hold Sleep and Home for 10 seconds: reboot the iPod Touch.