Monitor Off is a convenient gadget that will let you enter power save mode, by clicking it.
You can use this gadget as a companion to Sleeper, an app that will let you activate power save mode by using a keyboard shortcut, or Super Sleep, that will do the same but also forbid the exiting of power save mode unless a specific keyboard shortcut is used.
Run the .gadget file to start enjoying it immediately, or save it on your disk to install later.