Hyperspace Reactor

Hyperspace Reactors is a whole category of power sources - that vary in size and form, that according to human measurements are a perpetuum mobile. Since they produced massive ammounts of power from seemingly nothing (although it is believed that there IS some fuel, humans are just unable to establish what they are, at least for now).
Hyperspace Reactors have one massive disadvantage that far outweights their ability to produce energy without fuel. They pose an extremely negative influence on human mentality. While average people will be spared (at worse having a slight increase in nightmares), every single one notable mental disorders will soon end up expanded nearly infinitely, in most cases leading to tragedy.
As one can imagine, hyperspace reactors are very, very rarely used. They are researched intensely, and many wealthy collectors keeps them as a curiosity of sorts, but no sane person would ever try to power something up with them.