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.
Essential Mac keyboard shortcuts: Command-Z, -X, -C, and -V
Use the spacebar to scroll down in web pages on a 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.
How to take a picture of the screen (screenshot) on a Mac
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!
Essential Mac Keyboard Shortcuts: ⌘-B for bold
This is one you have to know: I rate it up there with Command-Z, -X, -C, and -V. And -P.
Use Control-Eject to bring up Restart/Sleep/Shutdown dialog on a Mac
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.
Essential Mac Keyboard Shortcuts: ⌘-I for Italics
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.