There is a difference between "free energy" devices and "perpetual motion" devices. We all know perpetual motion devices are impossible, as ellimist said, friction and conservation of energy laws.
My understanding of a free energy device however, would be a device that converts energy from its environment into a usable form. A windmilll is a free energy device, so is a solar panel and hydroelectric because none require any energy or fuel input by the operator. I believe "zero point energy" devices, if they are possible, would fall into this catagory as well.
The problem is, so far, no-one in the mainstream scientific community has been able to detect or prove the existance of the latent energy from the fabric of space-time that these devices are supposed to harness. There are many who claim to have working devices, some more credible than others, but none have come up with a stand alone independently testable device to prove their case. So in closing, maybe, maybe not, time will tell.