Most of us use our computers in a pretty consistent routine; after work, weekends, perhaps early morning (myself excepted; I suckle on mine 24/7). Therefore, keeping your 'puter at a high level of readiness (i.e., eating up kilowatts) all day and night long racks up electricity use that serves no purpose but to provide heat for the cat that sleeps on your keyboard.
This is the problem that the freeware program Edison fro Veridiem was written to solve. Based on the software written by Veridiem for large corporations, the program allows you to establish your schedule of use, then controls the power levels of your computer to keep it at the minimum current needed at the moment. For example, if you go nighty-night at midnight, your PC might drop into hibernation mode, waking itself up shortly before your alarm clock goes off.
How much can you save? Verdiem estimates that on average it can cut use per PC by around 410 kilowatt hours yearly. With electron juice now selling at almost $.09 a kWh, you could save around $40 a year. Why wouldn't you want to save $40 a year at no cost to you?
For those eager to reduce their carbon footprint (nice that such a desire is consistent with saving money, eh?),-- Edison will help cut back on your need for electricity, almost half of which, in the U.S., is produced by coal-burning generators.