Mac users, if you know only four keyboard shortcuts, let them be these four! Command-Z, Command-X, Command-C, and Command-V are what make the Mac the Mac.
When browsing web pages, tapping the spacebar scrolls you down one screenful. Shift-spacebar scrolls you back up. This works in Safari, Firefox, and Chrome. Way easier than using the mouse or trackpad.
Command-Shift-3 takes a picture of the entire screen. Command-Shift-4 takes a picture of whatever you draw a box around. There are other options too!
Bring up the Mac's Restart/Sleep/Shutdown dialog box, as if you'd pressed the power button. Much more conenient to do it this way.
Add Command-I to your list of essential Mac keyboard shortcuts. It turns selected text into italics. If it's already italicized, it turns italics off.