If Al Gore's documentary "An Inconvienient Truth" didn't scare you into lowering your carbon footprint, then maybe the idea of killing a living thing will set you straight?!
Winner of the 2007 Microsoft Start Something PC design competition, Ben Arent created the Energy Tree, a living tree contained in a large computer that monitors your energy efficiency. The tree's health depends on how much recycling you do and your household energy savings. The greener you are -- the better the tree does.
If guilt caused by destroying the planet doesn't motivate you, trying to explain to your kids why you let that poor tree die might set you on the road to a more eco-friendly life.