If you give many presentations on the Mac, chances are you're using Apple's Keynote presentation software. One cool feature in Keynote is the ability to demo something on your Mac without giving away your next slide or notes. When you're in the middle of a presentation, just hit the "h" key on your keyboard. This will hide the entire Keynote application and show your desktop.
When you press the h key, the Keynote icon in the dock will change and include a play button. When you want to resume your presentation, just click the Keynote icon in the dock.