Car services are a bit beyond my means, or perhaps I should say, I have no specific idea what car services cost because I've always assumed they are beyond my means. Which, I'm sure, they are. But enough about me.
Planettran has put the eco-twist on car service by putting into service a fleet of Priuses to ferry the swells around town.
They're not new, but I became aware of them only recently when I saw one of the vehicles waiting for a client and was able to talk to the driver for a minute. They started here and are based in Cambridge, but they have expanded to San Francisco.
If you use a car service, they do get better mileage than taxis, and certainly do better than stretch Hummers.
Contact info here.