Put it to sleep

On Wednesday Microsoft released "Why PCs Should Get More Sleep," which promotes the idea of using sleep (or hibernate) mode on PCs rather than running a screen saver. "To explain the dramatic difference power management can make...a typical desktop PC and LCD monitor sitting idle for a year (outside of business hours) would consume 632 kilowatt hours of energy — compared to 34 in sleep mode. That’s a savings of 598 kilowatt hours per PC per year." To illustrate it another way, the article indicated that a PC running a screen saver consumed as much power as a 100-watt light bulb, about $80 per year, dumping 1,350 pounds of carbon dioxide emissions in the atmosphere, "a major contributor to global warming."

In Windows Vista to enable/configure sleep mode go to the Control Panel > System Maintenance > Power Options. With Windows XP it is Control Panel > Performance and Maintenance > Power Options. On Mac OS X it is System Preferences > Energy Saver.