You answer a few questions then they give you some tips on how to reduce your carbon emissions. You can then plant one of the beautiful trees Jenny designed and then watch it grow.
One of the cool things they did on the site is build a WebSlice that let's you easily watch your tree grow in IE8. (WebSlices are a new feature in Internet Explorer 8 beta 1 that allow you to subscribe to part of a web page.)
Here's my tree (named "Chortle"):
Check it out!
(Disclosure: my team helped sponsor this project.)