PushMonitorOff is one such cool application which lets you turn off monitor quickly with a keyboard shortcut, but why would you want to turn off your monitor ?
Sometimes when you just want to turn off the monitor and keep the other applications running, it could be required as you may be leaving your seat in office for specific amount of time but you want the programs keep running.
This could be really good idea if you want to conserve some energy and save the environment around you, the default keyboard shortcut to turnoff your laptop or desktop monitor is Shift+F1