You already know that you can adjust brightness on your primary display by tapping the F1 and F2 keys (the ones with the brightness, or “sun” icons on them). But what if you have two displays connected? Maybe you have a MacBook plugged into a second display, or another monitor on your iMac; whatever. If you do, you may have thought you had to open the System Preferences to get to the brightness control for the second display.
Turns out, you don’t! If you hold down the Control key while tapping the brightness buttons, you can actually change the brightness on the second monitor!
(This was tested on Mountain Lion — no idea if it works on previous OS’s. Also I don’t know what’ll happen if you have a third display… if you have more than two, test it out and let us know!