When you're using your Mac to give a presentation to a group, the text or pictures on your screen may not be easily read by your audience. That's where OS X's built in zoom capabilities come in handy.

Go to System Preferences > Universal Access and hit the On radio button under Zoom. You can now zoom in by hitting Command-Option-= and out by hitting Command-Option-Dash (basically Command-Option and the plus or minus keys). You can tweak how much it zooms in or out in the options area under Zoom.

Now give that presentation in style.

Zoom is also useful for those with impaired eyesight.