Algodoo

Credit to Conundrumer for the original idea, way back. I discovered that a) the springs can have length 0 instead of negative length (though it doesn't matter much), and b) the engine is more powerful if the springs connect to the rotor instead of the little circles around the edge. The fact that there are two springs per circle doesn't make it more powerful, but it does make it more stable—it eliminates the unreliable direction bias this engine had.

I also built a similar gearing to Conundrumer's original that steps the engine speed down to a more manageable ~600RPM, and gives it fantastic torque—though like all spring engines, it has pathetic stopped torque and little stored energy, so it needs a buffer from the wheels, like a high-but-not-infinite-strength braking motor, or it will stall. You could also use a low-friction link in the drivetrain to either directly limit power or charge a flywheel.

Feel free to use it—I'm horrible at building anything more complex than this, and I'd be interested to see what it can do.