Check out TerraPass, an "eco-capitalist" venture. The premise is this:

Every car produces a certain amount of CO₂ annually. However, if we offset that production of CO₂ by reducing our production somewhere else by a commensurate amount, then the net effect of driving is essentially zero. In real-world terms, this can be achieved via the Chicago Climate Exchange, where greenhouse-gas production credits are traded. If we buy up credits and "retire" them, then we are increasing the real-world value of production credits and thus forcing companies to conserve more. TerraPass, the product of a Wharton professor and his students, does exactly this.

The Chicago Climate Exchange seems like a good idea, and it actually has some teeth to it. Although its membership is small, not even a thousand companies, it accounts for 230 million metric tons of CO₂ emissions, which is something like 4-5% of U.S. annual production. Not bad. And its emissions-reduction performance is also reasonable.

And the idea of "retiring" credits also seems relatively sensible. There's other organizations that do this; the other one on the CCX is Carbonfund.org, which is based on a similar premise to TerraPass; reduce your footprint by giving them money, which they'll use to retire credits on the Exchange.

But I vastly prefer Carbonfund to TerraPass. Why? Simple: marketing. TerraPass sells itself as a way to reduce the guilt of driving. First, your TerraPass footprint is based entirely on your car's gas mileage (which, by the way, underestimates your car's CO₂ footprint, since the energy returned on energy invested for gasoline is at least as bad as crude oil, which is 20:1) instead of a more comprehensive assessment of your total lifestyle. Second, their "product" is a sticker you can put in your car window, or on your bumper, showing what a good citizen you are. Third, all their press indicates that this is what they are offering to people.

Not to be unreasonably vicious about this, but people really should be made to suffer for the crime of driving. I say this simply because they need to be encouraged to stop, or at the very least drive 95% less. The last thing that we need is more ways to stabilize car culture, which is explicitly what TerraPass offers. (See their TerraBlog if you are unconvinced.)