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Zero-point energy isn't actually related very well to "free energy" machines, and it's an unnatural association between the two.

Attaining zero-point energy, in my opinion, is impossible outside of a total vacuum, which we aren't able to simulate or find, not even in outer space. That said, finding zero-point energy really wouldn't do anything for us except solidify the definition of the Kelvin temperature measurement, at least as far as I'm familiar with the concept.

Free energy is not possible. As the Second Law of Thermodynamics states, energy cannot be created nor destroyed, only change in form. Now, we've already established that matter is just another form of energy, and the two can be converted back and forth theoretically. It would be a great way to solve some energy shortage problems if used properly, as well as a great way to recycle excess matter (ie, trash). Still, it's all about the application that makes it dangerous, just like all other facets of science.

The concept you are referring to in your original post IS talking about free energy. Unlimited energy is a bit of a misnomer, since there will always be a finite amount of energy in the universe (once again, believing that matter is a form of energy).

Please further explain what you are talking about.

Yes, that would be useful, but I honestly don't think that kind of thing is feasible. There was talk of a machine that would generate energy through wind in the sky, powering its own propellor to do so, but as of yet it's never gotten off the ground, so to speak.