"The Sydney-based Easy Being Green says it will mitigate your cat's flatulent contribution to global warming for 8 Australian dollars, or $6. The same company could also make your granny "carbon-neutral" at 10 dollars a year, according to a report in the Australian newspaper last weekend.

Then there's Carbon Planet, another company cited in the article. If you are hopping on a short-haul flight between Sydney and Canberra, and feeling bad about the damage you are doing to the ecosystem, you can buy credits worth 23 dollars, for which the Adelaide-based company will guarantee to keep 1 ton of carbon dioxide out of the air for 100 years."
http://www.iht.com/articles/2007/02/22/bloomberg/sxmuk.php
linked from http://www.junkscience.com/