A rapidly spinning, slowly accreting magnetic white dwarf (or X-ray pulsar) in hibernation is expected to result in rapid spindown as a result of the stretching and reconnection of magnetic field lines, leading to particle acceleration at the magnetospheric radiusoutside the corotation radius, and the propeller type ejection of magnetized synchrotron-emitting clouds. This may explain the non-thermal (radio andγ-rays) emission seen from the unique nearby AE Aquarii. Moving to Galactic distances we show how TeVγ-ray observations of pulsar-driven supernova remnants (with well-measured synchrotron X-ray spectra) allow us to obtain a direct measurement of the average magnetic field strength in the nebula. Finally, GeV to TeV observations ofγ-ray blazars out to redshifts of ∼ 2 allow us to probe the intergalactic infrared radiation field, the Hubble constant and possibly the Ω parameter of the Universe.