New research by the NIMBUS lab at the University of Nebraska-Lincoln has demonstrated how a UAV could hover over electronics on the ground and charge them. This might be useful for distributed sensor networks that can't use solar energy to recharge batteries.
From IEEE Spectrum:
The type of wireless power that these quadrotors are beaming out is based on what's called "strongly coupled magnetic resonances." Basically, you've got two coils of wire: one on the quadrotor, and one on whatever you want to power or charge (we'll call this the receiver). The quadrotor drives a current in its coil, which generates an oscillating magnetic field. When the quadrotor gets close enough to the receiver, the receiver's coil starts to resonate with the magnetic field transmitted by the quadrotor. That resonance induces a voltage in the coil, which the receiver can use to power its electronics or charge its battery.
As far as applications go, the researchers suggest that this kind of system would be great for "highway messaging systems, ecological sensors located in forests, or sensors shallowly embedded underground or in concrete." UAVs would act as mobile power stations, zipping around and delivering power to sensors when necessary.