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It all boils down to what you mean by "free". Petroleum is "free" energy, just waiting to be harvested from the rocks. But there are catches, and probably a limited supply.
Tesla towers harvested low frequency oscillations of Earth's magnetic field. Not something you want to drain. "Space elevators" harvest Earth's rotational momentum for cheap (per launch) entry into space. But slowing rotation is not to be taken lightly.
Any source of "free" energy follows the same mathematical principle. Something is rearranged in the universe, increasing total entropy.

Not widely peer reviewed, but people built devices using Davis Mechanics: F = ma + Da' where D is the Davis constant and a' is change in acceleration. This also describes high a' events like heavy metal munitions striking a hardened target like a tank.