I came across a neat and simple piece of software some time ago, called Local Cooling. It maximizes your computer's power saving options, and puts a little indicator on your screen to tell you how much energy you've saved since installing the program, in terms of gallons of oil, kiloWatt hours of electricity and trees. It is free software, and I have it installed on both my home and work computers. It's amazing how the energy savings add up over time, and it is neat to see the savings converted to trees - it makes it more meaningful to me in those terms.
By installing this software I've also noticed that my old CRT-monitor at home sure uses more power than my flat screen at work. But I can't really justify buying a new monitor for home when there's nothing wrong with it.