History Brush (Keyboard: Y)

The history brush lets you paint back in time. Photoshop keeps track of all the moves you make (well, 50 by default) and the history brush lets you paint the past back into the current photo. Say you brightened up the entire photo but you wanted to make a certain area look like it did before you brightened it, you can take the history brush and paint that area to bring back the previous darkness.