There is an emerging world of browser-based technologies that are extending functionality in hitherto unforeseen and exciting ways. A good example of this is Real Costs for Firefox. Like most of these sorts of plugins on Firefox it uses Greasemonkey.
Real Costs adds a lovely visual display of carbon emissions data to your view when you visit popular travel websites and airlines (mainly American at this stage – see their development wiki) allowing you to compare emissions across other modes of transport and assess your carbon credit liability.
The objective of the Real Costs is to increase awareness of the environmental impact of certain day to day choices in the life of the Internet user. By presenting this environmental impact information in the place where decisions are being made, it will hopefully create an impact on the viewer, encourages a sense of individual agency, and provides a set of alternatives and immediate actions. In the process the user/viewer might even be transformed from passive consumer to engaged citizen.
The project is supported by Eyebeam and Rhizome amongst others.