David Bornstein's WC3K contribution about Terrapass elicited an interesting response. Eric Carlson, president and co-founder of Carbonfund.org, wrote to remind us that his organization (a) is a non-profit (unlike Terrapass) and (b) charges less per ton of CO2 than other carbon offset services.
(And how do they do it? VOLUME! No, seriously -- they combine multiple offset donations to be able to purchase and retire large blocks of carbon credits from global carbon markets.)
Many thanks for the update and inclusion. And thank you to the many people who have visited Carbonfund.org from Worldchanging.com and offset their climate footprints.