Time Magazine named CarbonRally one of its 50 Best Websites for 2008: 

"CarbonRally posts challenges, like cutting your shower time by two minutes or line-drying your laundry for a month, to motivate you to reduce your carbon footprint. Never cleaned your refrigerator coils? Neither have we. CarbonRally shows you how to do it and explains why it helps. You can compete on your own or, for added incentive, as part of a team. For extra enviro-credit, check out Zwaggle, where you can donate used clothes, appliances, books and other merchandise to members, then "buy" stuff from other Zwagglers with the points you earn."

I think this is a great idea.  Try one of the challenges on CarbonRally and see if you can handle the competition!