TerraPass
I just signed up for a great program called Terrapass. It’s a CO2 reduction project. It allows you to purchase credits in clean renewable energy to make up for the emissions from your car. It’s incredibly affordable. For only $50/year, I purchase a share of renewable energy that will replace energy generated from polluting fossil fuels and reduce CO2 emissions by 12,000 lbs. That’s way cheaper than buying a hybrid. 
Some related sites, that also allow you to purchase “green” credits: Native Energy, Green Tags and Carbon Fund.
March 12th, 2006 at 8:23 pm
Hi Matt. Can you specifically pick the clean renewable energy that are going to buy with your money?
March 13th, 2006 at 6:19 pm
Most of the programs have a set mixture of the energy sources. Most use wind with some combination of solar or methane. Carbon Fund is a little different. They actually buy CO2 credits from the Chicago Climate Exchange, and then retire the credits so that the CO2 cannot be produced. I actually think I will switch to that next time. Also, Carbon Fund is a not-for-profit, so I think it is tax deductible. No matter what you choose, you can’t lose as they are all great programs.
March 28th, 2006 at 9:56 am
i was under the impression terra pass purchases credits from the chicago climate exchange (CCX) as well, so do other groups like cleanairpass.com, and drive neutral. I noticed many offsetting groups are non-profit, but terrapass and cleanairpass are not. Some of the non-profits charge multiple times more per tonne though than the ‘for profit’ groups. I personally think whether profit or not, the same work is being done. Some for profit groups have teamed up with non profit orgs and can offer tax deductible reciepts. yes, i spend way too much time looking at this stuff.
anyway, my whole point is that i though terrapass purchases from the CCX too.
http://www.terrapass.com/portfolio.html
if you go to that link, you can read in the “Project” column that the CCX was involved in the purchase.