I found this the StepGreen site (http://cmu.stepgreen.org) last semester but have not yet taken the time to actually use it. It was created by CMU students from the HCI. Looks promising—you commit to actions and can even share your actions with others via Facebook (utilizing subjective norms, which are a key part of behavior change).
One thing I wonder about is how easy it will be to remember to go back to the site to log actions. According to the FAQs, one must log into the site to reconfirm repeated actions in order to view your progress. I wonder how many repeat visitors the site gets and what the incentive is to keep people using it. I am planning to give it a try and I’ll report back!