I use Green Cabs a lot for driving around the city.
My initial hesitation to call them was based on the thinking that if they were marketing themselves as 'green' therefore they must be expensive. The opposite is true - they are really competitively priced.
They also have some of the nicest drivers I have met, most of whom seem really passionate and knowledgeable about their vehicles.
It does make you feel a little better to be using a more environmentally friendly vehicle.