Just this afternoon, I finally got around to putting the TerraPass I bought a few months ago onto my car.
This is a very cool idea. You tell them what kind of car you have and how many miles per year you drive it. Then they sell you a TerraPass that offsets the carbon dioxide emissions from your car for one year. This is done by investing the cost of the passes in projects that reduce carbon dioxide emissions elsewhere, mostly renewable energy capacity. They calculate how many pounds of carbon emissions are eliminated based on your investment. It's a lot more affordable than you would think -- it cost me $30. My car is relatively efficient, and I don't drive much, but it's still relatively cheap even for people with more emissions.
Obviously, the calculations involved here are inexact. The more troublesome part is whether the carbon emission reductions your investment creates really wouldn't have happened without your money. I guess as an economic matter, every additional increment of investment expands a given industry. But every bit of money invested in, say, wind farms also has an effect on the decisions of others to invest. So that complicates things, but I can't imagine a scenario where TerraPass investment wouldn't decrease net overall carbon emissions. (Unless people feel less guilty about driving their cars as a result, and drive more -- but I don't think that's all that likely on a large scale.)