Rigs of Rods

Rigs of Rods is a free multi-simulator (aviation, water and rail transport, and others) that uses soft-body physics to simulate the movement and deformation of cars. The game is built using a special soft-body physics engine called Beam, which simulates a network of interconnected nodes (chassis and wheel formation) and allows you to simulate deformable objects. Collisions with walls or terrain permanently deform the vehicle.

In Rigs of Rods you have no main objective, but scripting support implies mission support. Multiplayer supports up to 16 users with collision simulation on the same field.