Rigs Of Rods (also known as RoR) is a completely free simulator of cars, boats, planes and trucks.
Rigs Of Rods is outstanding because the chassis and wheels of the vehicles are simulated in real time as flexible objects, giving this simulator a behaviour that is very faithful to reality. The flight simulations are based on the shape of the wings, the dimensions of the plane, the wind resistence, etc. The boats are also highly realistic, and the detail and complexity of the hulls make them move like real boats.
From the Rigs Of Rods website you can download hundreds of vehicles developed by the community of this game.