Since the event emissions calculator is the most unusual (TerraPass does vehicle carbon offsetting, Go Zero does flights), I took it for a test. I figured out the CO2 emissions for a Thanksgiving get-together. I told the site that 15 people would drive less than 100 miles, and 5 would drive from within 500 miles, and there would be one hotel stay (let's pretend you've got a big house for the out-of-staters, but it's not big enough for Uncle Harry, because, well, you know). DrivingGreen said the holiday dinner would put out 2,200 pounds of CO2, which I could offset for eight bucks. That seems awful fair, doesn't it? Add in a short flight and the numbers jump to 4,400 pounds and $16. DrivingGreen estimates an average 2-day event with 250 people - including ground travel and hotel room power (no flights) emits approximately 72,000 pounds of CO2, offset-able for $260.
The site is obviously not precise. It assumes a two-night hotel stay, for example, and the option for distance includes a "100 mile/200 kilometer" category, and those two are not equivalent. But hey, I'm not complaining.
DrivingGreen.com uses the money it raises to help selected farmers fund digesters that convert animal waste into renewable energy that is then used to power their farms without any fossil fuel. The farms are Colorado Pork LLC in Colorado, New Horizons Dairy in Illinois and Holsum Dairy in Wisconsin. DrivingGreen is run by AgCert International plc, which was "founded in 2002 to produce and sell reductions in greenhouse gas emissions."