Free energy is possible in the sense you can create harvesters that will gather energy that doesn't require you to pay for it. Perpetual Motion Machines and Zero-Point Energy Machines are pretty much impossible. Literally impossible in all conventional senses, but I'm not just going to assert this. Let me actually be constructive.
Energy is simply Work, that's it. Energy we can use is Mechanical Energy. This energy has two states, Kinetic Energy and Potential Energy. If you throw a ball into the air, the ball has low potential energy and high kinetic energy. When the ball stops rising and is about to fall back to Earth, it has low kinetic energy and high potential energy. That's how it works, and any device you make needs to conform to this principle.
The next thing to consider is what exactly you're harvesting. I imagine in this sense you are trying to harvest electricity to use. So Electrons transfer power by Photons. It's not exactly true, but think of it like that. So Electrons repel each other kind of like Electron A throws a medicine ball called Photon towards another Electron, called Electron B. So Electron A is pushed back by this toss and Electron B is pushed back when it catches the Photon ball. This causes the repelling forces or the interaction of the Electrons themselves.
Now you are in luck because Photons are mass-less, but Electrons are not. Quarks are actually pretty heavy and as such a perpetual machine will be adding energy to a system, which will be adding mass, which will be adding gravity, which will soon be incapable of being picked up or moved, eventually collapsing into a black hole.
Zero Point Energy is the ground state any quantum field can reach. If you were to reduce a molecule to Absolute Zero, it would still have energy simply from being in this Universe. Removing this ground state, or Zero-Point Energy would remove that molecule or atom from this Universe. You left a link demonstrating that scientists had harnessed Zero-Point Energy, but you are wrong. What the scientists demonstrated was that these virtual particles popping in and out of existence were able to be brought into our Universe by exchanging Kinetic Energy to the particle's ground state, causing it to have a more energetic state. Also, they used electrons to produce a photon, which is mass-less and easier to deal with.
Now, let me undo everything I just said. Perpetual Motion Machines are possible, and getting Free Energy from the Zero Point field is possible, sort of.
I think it was Hideki Yukawa who figured this out, but he said that there must be some sort of particle holding protons together. He predicted the mass and charge of these particles and he called them Mesons. Eventually the name would be called Pions and latter Muons. They found that the Heisenberg Uncertainty Principle explained how much you could borrow from the void and for how long. So you could borrow a Muon to keep protons together for a very short period of time, and in a very short distance. I think some where in the magnitude of 1 x 10 to the negative 14th power meters. Some where around there.
One thing the early scientists saw in the Cloud chambers (I can't remember if it was cloud chambers or bubble chambers, it was around the 30's and 40's) was that the Ion Trail was straight like an electron. So long story short, the Muon was able to turn into an Electron.
So the device you're going to make will need a Photovoltaic receptor or cell that will produce a charge when exposed to photons. A radioactive element that will shower an area of matter with particles that will produce an ion trail. Then a mysterious 4th step, and PROFIT from your endless supply of true Free Energy that didn't affect the general mass of the Universe because your Muons decayed into beautiful electrons and then winked back into the void but not before producing photons from your Zero-Point Energy circuit from that science article. Hurray. Or make a black hole at the center of your TARDIS and soak up all the temporal rift energy.
On another note you could make use of your own body to produce energy with flexible circuits or Rochelle Salts connected to a Miller Solar Engine, minus the Solar part.
Just know that I'm in your corner cheering you on, so long as you understand that the Universe is a closed Adiabatic System when it comes to our technology. Until you start travelling to other dimensions, we live in a closed system, and you are bound by the conservation of energy. Your machines will break down eventually unless you put more energy in. You will always have a net loss.
- The Grigori
P.S. If you have the means, look into making Electrets. They will gather static charge, but could be useful in strange ways. Probably a good place to start.
edit on 16-10-2012 by Grigori because: Trying to think of more helpful ideas for devices.