If you're looking to save money on long-term parking fees while also making some extra cash, startup FlightCar has a deal for you.

Instead of plunking down cash to pay for parking (if you're leaving town) or for a rental car (if you're arriving), you can rent your car out, or if you need wheels, rent someone else's car.

Seattle is the fourth city that the San Francisco-based startup has moved into--you can already use FlightCar in Boston, Los Angeles and San Francisco.

Here's how it works: Travelers leave their car at the airport for free, while others pay to rent the idle cars. Mileage is limited to 100 miles per day, the rented cars must not leave the country or be involved in any illegal activity, and renters can't smoke in the vehicles. Liability and comprehensive insurance are covered, and anyone who has committed a major driving violation in the past three years won't be allowed to rent.

If you're under 21 years of age, expect to pay an additional $25 per day, while those who are 21 will pay $10. That's on top of the rental cost, which might be as low as $15 per day.

If this idea continues to take off (pun intended), it might make for a better travel experience. Departing travelers will save massively on parking fees while renters can undercut the prices charged by rental-car companies. Free pickup and drop off is included, as is a car wash.

FlightCar already has 30,000 members in Boston, Los Angeles and San Francisco, and the startup has been in business for just 18 months. It hasn't been all rosy, since the company is battling San Francisco's airport over whether it's a peer-to-peer car-sharing service or a traditional rental agency. Since the Seattle location isn't on airport property, the company says it expects no problems in that regard.

If the demand continues, FlightCar may be coming to an airport near you.

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