Sometimes you want to change your ruler increments in Photoshop from, say Millimeters to centimeters or even pixels. Many people will find themselves going to Preferences>Rulers and Units to change this figure - but that sounds like a-lot of unnecessary clicks to me!
To change ruler increments, or even just check what they are currently set to, just CTRL+Click (right click) on either the vertical or horizontal rule at the edge of your window, and a handy little pop-up menu comes up with all the different measurement types.
Just as a litle reminder, if you don’t have your rulers visible, the keyboard shortcut is Apple+R.