A bit of searching reveals several schemes for getting energy associated with Tesla.
One involves using the temperature difference between the earth and space. That amounts to a theoretically ordinary, and extremely impractical, heat engine, although Tesla seems to have thought it was something else.
Another involves some sort of extraction of energy from a universal medium, a classic crackpot idea that has absolutely no physical justification.
Finally, there's the idea of using the potential difference between the earth and the atmosphere (the same voltage that drives lightning) as a power source. In principle that's possible, but it would require an enormous expensive apparatus to extract relatively little power. Since the ultimate driving source for that power is the sun, which deposits little of its energy in these modes, conventional photovoltaic devices, concentrated solar thermal devices, and wind turbines are far more practical ways of harvesting large amounts of solar energy. (Wind is also driven by the sun's energy.)
So far as I can tell, the Tesla cult is a way of getting naive people to pay for worthless "plans" and other gimmicks.
(published on 02/17/2012)