If you rely on Power options in Windows to enter your system into power-saving mode after specified time to save energy and you can also use on/off button on Monitor to save more energy but Sleeper is a freeware portable tool lets you send your monitor to power-save mode with a keyboard shortcut.
Download and extract Sleeper archive contents to a folder. Create a shortcut to Sleeper executable on desktop,assign a keyboard shortcut to it which ever you feel comfortable and not interferes with other applications by right-clicking on shortcut icon>select Properties>from Shortcut tab.
Sleeper doesn’t have any user interface it sends “sleep” command to Windows and closes it self automatically, assign a shortcut to the program and use it to put your monitor into power-saver mode. Press any key on keyboard or move your mouse to exit power-save mode.
Sleeper works on Windows XP/Vista/7.