As someone who travels to Philadelphia often enough by rail and air, I joined the PhillyCarShare.org system a couple of years ago. They have cars parked at 30th St. Station and many other locations throughout Philadelphia, including near many SEPTA stops. They even have a deal where they pay your SEPTA fare.
But, what started as a fleet of shiny new cars including many nice models, degenerated into dented and dirty cars that have not apparently been replaced. At first the cars also included a gas card. Part of the attraction was that the rental fees included gas and insurance. Later they removed the gas cards due to vandalism, and required a rebate form for gas reimbursement. Then a few months ago PhillyCarShare started charging a monthly membership fee. At that point I decided to quit. I don't go to Philadelphia often enough to justify the monthly fee.
Nevertheless, PhillyCarShare remains one rental option at 30th St. Their reservations are done online or over the phone, and you need a key fob that comes with your membership to start the car. They are not in the rental car office at 30th St., but they have some cars parked outside.
Zipcar is also in Philly and may be a better deal. They allow you to rent cars in all their cities, so if you join in say, San Francisco, you can use their cars in Philly too.
