It's amazing how buried some of these are. Nothing tops print screen for me - Cmd + shift + 3 for the whole screen or Cmd + shift + 4 for the selection option...
My absolute favorite is cmd + shift + 3 - release - space to take a shot of a single window. Maybe it's the release part between 3 buttons and pressing space, but that shortcut is always wired to use for me.
I didn't knew this one, it's awesome, but it's with 4 instead of 3 :)