You bet. Just took a Compressed Natural Gas car service to JFK. Granted, I have to offset the big old jet flight I took, but at least my C02 to/from the plane was basically nil. For a good overview about this alt fuel, check out About.com .
Kudos to US Limo for working with LimoGreen and incorporating several CNG vehicles into their fleet. And what I really thought was pretty cool was that their drivers are "pumped" about it as well. One of them got out of the car to show me where the tank was and explained about the almost zero emissions. He almost went as far as to put his mouth by the tail pipe to prove it, but I held him back. (Granted, some don't want the calls for rides requesting it because the fuel stations are few and far between still. For private cars there are still only two in Manhattan.)
I'm no scientist, and I tried reading a bunch of stuff on CNG, but you're better off comparing for yourself. What I take away is that it is certainly a lesser evil (less "particulate emissions" if you must know), and a great place to start is on all city vehicles in part because the fuel tanks for these babies take up a lot of storage room in passenger car trunks and conversion kits are available but pretty costly.
BUT - I've always been of a mind that the kind of enthusiasm that driver showed is what we need (hence my blog!) to keep the word spreading. T Boone is doing a good job of evangelizing CNGs with his Pickens' Plan. We can't afford to fall down on the job right now.
Oh, and can we get a few MORE cars into car service fleets? Start requesting them if you're going to the airport and create some demand for good.