semanticpool

… when booking your next flight? ;) There is a great project, which doesn’t focus on the money you pay for the flight, but the most important costs:

Real Costs is a Firefox plug-in that inserts emissions data into travel related e-commerce websites. The first version adds CO2 emissions information to airfare websites such as Orbitz.com, United.com, Delta.com, etc. Following versions will work with car directions, car rental, and shipping websites. Think of it like the nutritional information labeling on the back of food… except for emissions.
(www.therealcosts.com)

Also check out the official anouncement by the creator.