Source Chariots was my winning entry in the 6/28 - 7/11 Innovation Challenge at HL2World. It is a multiplayer map where two players can race around a canyon track set in the desert. The chariots themselves where the highlight of the map, as they are 100% driven by Source's physics engine.

All of the thrusting is done dynamically, unlike other physics vehicles which simply turn thrusters on and off. All of the throttle is controlled by the turbines in the front of the engines. While you can't see it while driving, the turbines spin and control the speed at which the racers go. The throttle gauge is a visual representation of the 0-100% scale at which the turbines are spinning. This scale is then applied to the forward thrusters, and raises and lowers as the turbine spins faster and slower.

There are no static phys_constraints in the entire system. The engines are held together by a set of springs and slide constraints, and the engine connects to the pod by a group of length constraints. The cockpit is literally dragged behind the engines by their sheer force.

You can read the original release thread here.