Rigs of Rods ("RoR") is a now open source, freeware, multi-simulation game which uses soft-body physics.
Rigs of Rods is based on a specific soft-body physics engine called Beam. Beam simulates a network of interconnected nodes (forming the chassis and the wheels) and gives the ability to simulate deformable objects. With this engine, trucks flex and deform as you ride the terrain, which can be based on real terrain elevation data. Crashing into walls or terrain can permanently deform a vehicle.
Get it at www.rigsofrods.com
Remember, it has some bugs when playing online so if it crashes, don't worry.