Someone just posted something about harnessing 'zero-point energy' on the forums and I felt I needed to say something. If you want the quick of it, you can just read the title. For the more detailed explanation of why most respectable people come to that conclusion, continue reading.
First, let's talk about what zero-point energy
is. While the linked article is certainly a good read, I'm going to shoot for a more down to Earth explanation for those who don't wish to bother.
If you set a ball onto the ground, it will roll around until it eventually reaches the lowest point in its local area. For most practical purposes, you might say that the most energy you would normally get out of the system is the difference in potential energy from where you set the ball down and where it finally rolled to.
However, if that ball is next to a giant cliff, a little bit of extra energy may go a long way - just push the ball over the cliff, and you get that much more energy.
Taking this allegory back into the quantum mechanical world, you can construct an equation so that this cliff is infinitely tall. You can actually get more technically correct by adding an opposite infinity sign, allowing you to pick whatever number you like for the natural energy density of the Universe.
The wise will see this situation for what it is - our understanding of physics is not complete. This is actually one of those situations in we know both the question and the answer, but not the means to get from the question to the answer - the cosmological constant
Which amounts to about .6 joules per cubic kilometer. For the purposes of generating energy, this is not a particularly useful density - fuels like gasoline are measured in megajoules per liter, with a liter being one trillionth of a cubic kilometer. Fusion energy has more than a million times the energy density of gasoline.
That's not to say that vacuum energy has no use or will have no use. Half the point of science is to find clever ways to abuse odd facts and indeed, we may already have
But we're not getting infinite free energy out of it.