I like it, but I would have to do homework to make sure the money is being used as claimed.
I calculated my car in all 3 and got about 6 Tons CO2 per year @ 20,000 miles per year in a stock configuration. That came out to a $30 per year contribution for my car (EDIT : The Terrapass was $80, so it must have a different algorithm). I can now use that as a base to calculate my actual CO2 output (but this is still only a "smarter" guesstimate). For example :
(20,000 Miles / 31 MPG)*X = 6 Tons
=> X = 6 * ( 31 / 20,000) = 0.0093
Now, in actuality I am getting 38 MPG, so I can plug that in as follows :
(20,000 Miles / 38 MPG) * 0.0093 = 4.89 => 4.89 Tons CO2
The "buycarbon" website was cool because it allowed you to split your city/freeway driving estimates, so maybe it takes into account idling.
What is interesting is that they seem to be using the old EPA MPG standard, which means that normal drivers are probably getting worse MPG and therefore actually should pay more.
PS - I REALLY liked this part of the buycarbon website :
I like the concept, but I'd be inclined to put a jar on the fridge labelled "solar project", "kill-a-watt" or something like that, and feed it instead. That way you know exactly where the money's going, and it becomes personally productive.