Well, in reviving this aborted thread, I've often wondered what would happen if one were to run a wire up a mountain and connect it to a large plate at the top to collect the ~100 volt/meter atmospheric gradient, then connected that at the base of the mountain to an electrostatic motor, grounding the opposite set of electrodes.
According to Richard Feynman in his lectures, the atmosphere from top to bottom has a gradient of 400,000 volts, and a total worldwide current of 1800 amps. That's 7.2 MW of power continually being carried to ground.
Assume you have a 100 square meter plate (10 m x 10 m) and an altitude difference of 500 meters, you'd have 50,000 volts at ~200 microamps, which is ~10 watts. That's enough to drive a pretty substantial electrostatic motor.
It's a very small current density, only a few microamps per square meter, but with a large enough collecting plate and a high enough altitude, it'd be a pretty substantial current.
Where it gets interesting is when we finally get around to building space elevators... then we've got a direct line to the top of the atmosphere.