In early September, Betavine organised a 24-hour hack event called EcoMo09, with an environmental theme. Unfortunately I couldn’t stay the whole time, but I went along as an AMEE representative to support other people using our API during the event. In the quiet periods, I managed to come up with my own hack, which used GreaseMonkey and AMEE to add carbon emissions to Google Maps.
You can even get the code and try it yourself. It’s very basic, and actually most of the work was dealing with Google Maps and making Greasemonkey behave properly - the AMEE bit, getting the carbon value, was dead easy.