ZipCars

ZipCar is a company with a similar idea, but it works like a more of a traditional car rental - you prebook the car and park-back where you picked it up.
The idea that you can park at a different location that you started seems to be unexplored.

Part of their service that I find particularly interesting is their removal of the need to deal with car-rental staff. It's self service all the way - no human hassle. Apparently, this is a key feature of Car Sharing. Another convenient thing is that insurance and gas is covered, as you're not using the car for such a long period of time, compared to traditional car rental.

Also, they're based mostly in the US, only having a few locations outside the states: in major cities in Canada & Britain. Not sure who their competition is yet.
They've a very green theme - from their colour scheme to their environmental friendly ideals.

City Car Sharing (CCS) also has a very extensive paper on car sharing. (18 page PDF)

Going back to the idea that a car can be parked at a different location that it is picked up from, an issue has come up: how do you maintain a stable number of cars in each location.
If tons of people head into office areas during the morning that means the car-lots near public transport hubs will be emptied, while the lots in the commercial districts will be overloaded.