Three young entrepreneurs from California have come up with an idea that could earn you free parking at the airport. What's the catch? You let them rent out your car while you're gone.The start-up, called FlightCar, is the brainchild of three barely out-of-high-school business-minded individuals. The idea is to realize the possible synergy of people arriving for a flight with a car they need to put somewhere, and people arriving on a flight who need to rent a car.The benefit to the car owner is that his or her vehicle is whisked away from the airport upon their arrival and then given back to them at the curb upon their return, free of charge. The car is cleaned and gassed up to pre-trip levels, and the owner even gets a gas card worth at least $10.How about the renter? That person, who in this case wouldn't be a renter so much as a member of a car-sharing service (like ZipCar or RelayRides ), gets a vehicle to use while in town at a much cheaper rate than a normal rental car. One example given by Boston.com, which published an article on the start-up recently, was a Volkswagen Touareg that FlightCar could rent for $30/day, compared to a BMW SUV from Avis that cost $210/day.FlightCar is still in its infancy and only operating at the San Francisco International Airport, but that shouldn't dissuade you from participating as either a renter or owner. Each car is covered by a $1,000,000 insurance policy for liability, comprehensive collision and theft, and renters must even meet eligibility requirements. The company also only accepts cars with less than 150,000 miles that are model year 1999 or newer. Plus, the selection looks more interesting than what's offered at your average rental car counter, including a nice 2003 Mercury Marauder that's available to rent next week.