An online CO2 converter built by David Kjelkerud, Henrik Berggren and Jorge Zapico at Ecomo09, an environmental hacking day in London. It is designed to help you more easily understand what a kilogram of CO2 really means.
You can convert CO2 amounts to other units such as bottles of beer or compare different emissions for instance how many apples are equivalent to a litre of milk. The data used is mostly localized to Sweden.
David and Henrick also built http://citysounds.fm/ the web app that lets you listen to the latest music from cities around the world.