This is a list of free games utilizing a physics engine to simulate gameplay.

--WARNING--
SOME OF THESE GAMES ARE MEANT FOR THE EASILY AMUSED.

1.
Armadillo Run
- http://armadillorun.com
-- Use cloth, rope, metal, rubber and rockets to get the armadillo into the target. It's the full game in the demo, just minus the next 50 levels.

2.
Orbiter
- http://orbit.medphys.ucl.ac.uk/orbit.html
--The space flight sim, with a fully streamed and uninterrupted solar system
Newtonian physics (Don't recommend playing with those turned on.)
Not graphically the best, but very cool. All of it is to scale, too.
It also has a large learning curve-as with the regular Flight Sims. It's just that this one is free.

3.
Penumbra
- http://www.penumbragame.com
A very fun, free and dynamic game. It also has some scary parts.

4.
Sodaconstructor
http://sodaplay.com/creators/soda/items/constructor
--"Sodaconstructor is a construction kit for interactive creations using masses and springs. By altering physical properties like gravity, friction, and speed, curiously anthropomorphic models can be made to walk, climb, wriggle, jiggle, or collapse into a writhing heap."

5.
Ski-Stunt
http://www.cs.ubc.ca/~van/papers/skistunt.html
--An addictive skiing simulation game.