Car owners can list their car on flightcar and in return get free parking while away, a car wash and pickup/dropoff service curbside at the airport. Renters can rent cars from flightcar and be picked up at the airport. The cars available for rental are more varied and potentially nicer than those available from traditional rental agencies. Its a win-win. This video explains a bit more about how Flightcar works:
Flightcar has been operating at San Francisco International Airport (SFO) since February and has listed about 650 cars and had 1000 reservations. Now, San Francisco residents who leave their cars with flightcar while traveling to boston can rent a flightcar on the other end, effectively enabling carswapping.
With flightcar expanding and Zipcar launching some airport locations, the options for airport-based carsharing are really expanding.