Sleepytime

For a quick-sleep keyboard shortcut, hold down Command-Option-Eject for two seconds. If you're looking for a keyboard shortcut to put all displays—but not your Mac itself—to sleep, press Control-Shift-Eject. To get that handy Restart/Sleep/Shut Down/Cancel dialog (the same one you got by pressing the Power button on older Mac keyboards), just press Control-Eject.

It's also possible to put your display (not your whole Mac) to sleep in an instant whenever you drag your mouse cursor to one corner of the screen. Go to System Preferences > Exposé and Spaces > Exposé, and set one of the Active Screen Corners to Sleep Display.