Get two metal plates close enough together and vacuum energy will push them together. This is because the plates block out the light waves whose wavelengths that are too big to fit, in whole multiples, between the plates. Eventually you have more waves bouncing around on the outside than on the inside and the plates will get pushed together by this difference in light pressure.
The tiny force was measured in 1996 by Steven Lamoreaux, now at Los Alamos National Laboratory. His results were in agreement with the theory to within the experimental uncertainty of 5%.
It is doubtful that this can be tapped, and if it could be tapped, it is not known what the secondary consequences would be. Plus it also takes more energy to seperate the plates than could have been obtained by pushing them together.
Remember that this is our lowest energy point. To get energy out, you presumably need to be at a lower energy state.
Another recent idea is to replace the metal plates with plasmas. The idea is that instead of having to pull the plasmas apart, you allow them to collapse, and create a new set of plasmas, with which to start the process again. This theoretically could allow you to extract energy from the system. But this is, at present, no more than a theoretical exercise.