DURING THE LANDING ROLL THE AIRPLANE BEGAN TO VEER TO THE LEFT OF THE RUNWAY CENTERLINE. THE PILOT ATTEMPTED TO CORRECT THE DEVIATION BY APPLYING RIGHT RUDDER AND RIGHT BRAKE, BUT WAS NOT SUCCESSFUL IN STOPPING THE AIRPLANE FROM DRIFTING TO THE LEFT. THE LEFT MAIN WHEEL WENT OFF THE RUNWAY PAVEMENT INTO SOFT, WET GROUND, WHICH RESULTED IN THE AIRPLANE GROUND LOOPING AND STRIKING TWO PARKED AIRPLANES.