"Electric and magnetic fields and other fields are subject to a version of the Heisenberg uncertainty principle: it is not possible to have a state in which a field, and the rate at which it is changing, both vanish. Consequently empty Space, even far from any matter, is permeated with continually fluctuating fields. The effects of these fields are very weak under ordinary circumstances, but they can be measured - for instance, by observing a force between parallel metal plates due to the change produced by these plates in the fluctuating electric and magnetic fields in the Space between the plates. This is known as the Casimir Effect, and has been studied experimentally and theoretically for many years." [Steven Weinberg]
The Casimir Effect is a quantum phenomenon where two uncharged, parallel metal plates placed very close together in a vacuum attract each other. This happens due to quantum fluctuations in the vacuum, showing that even empty space has energy.
See Also
12.30 - Thermal Radiation and Thermal Vacuum or Cold
Keely - Electricity from Space
Kellner - Electricity from Space
Space Charge
Tesla - Electricity from Space
Zero Point