Markets in Everything: Private Car Network
"Request a car by telling Uber where you are. Text us your address, or use our iPhone or Android apps to set your pickup location on a map. Uber will send the nearest driver to pick you up, and text message you an estimated arrival time. Cars usually arrive within 5-10 minutes. Your licensed professional driver will park curbside in a sleek black car. Uber will text you again when the car arrives. Hop in the car, tell the driver your destination and you'll be on your way."
It seems like a variation of Zipcar, since you register in advance with Uber, and your credit card on file gets charged automatically for each ride, and no cash is necessary.
Obviously, the taxi monopoly in some cities like San Francisco is not happy about the competition from Uber (formerly called UberCab), and they got the San Francisco Metro Transit Authority (SFMTA) and the Public Utilities Commission of California to issue a cease and desist order against the startup in October.