I gave my brother a Terrapass for Christmas this year, since he's kind of hard to shop for, and he owns a Jeep. The idea is that depending on what kind of car you have, and how much you drive it, you give them some amount of money that they invest in clean energy projects. The amount is supposed to be enough to reduce industrial carbon dioxide emissions by the same amount that your car will contribute in a year.
It's an interesting version of carbon credits for individuals, and even more surprising is that they are trying to make a profit doing this - I'm curious to see if this kind of venture takes off.
I found the idea through worldchanging.com here, and they also point out another way to offset your carbon contribution via a non-profit, and not just tied to your car, at carbonfund.org - this is particularly interesting since as their page on your impact notes, you contribute towards CO2 emissions in more ways than just driving.