What about the macbook air?

Tap the power button, then the first letter of the choice buttons, then press enter. You can also just hit enter after the power button to select the default (which is shut down).

Another combination: Shift-control-power combination will sleep display.

Go to System Preferences -> Desktop & Screensaver (or Mission Control)

Click to enter Active corner settings

Here you can sleep the display (or start screensaver) whenever you move the pointer to a designated corner.