It's a dream of many people: free energy, cars running on water, vacuum energy, over-unity machines, etc. Here's a dream that may (perhaps) one day come true. If we can find a friendly, rotating black hole within reachable distance and with a hospitable environment around its equator, we can perhaps colonize the space around it and have clean "free" energy for our colony!
Pictured below is a design based on Sir Roger Penrose's black hole energy extraction process. It shows a mega-city built on a rigid structure around the equator of a massive, spinning black hole. The city is at just the right distance from the hole so that the local gravity is at a comfortable 1g and that tidal forces and inertial frame dragging do not affect the structures adversely.

Credit: "Gravitation", Misner, Thorne, Wheeler (MTW), fig. 33.2
The city's garbage is processed at the top left by dumping it into suitable containers and pushing the containers at a carefully chosen angle towards the black hole. Due to frame dragging, the containers swing around the black hole in its ergosphere[1], and so "steal" some of the black hole's angular momentum.
Just before the container starts to leave the ergosphere, the garbage is ejected towards the hole, to be "swallowed" by it. Due to black hole dynamics, the garbage instantly attains negative energy. The negative energy entering the black hole causes its rest mass to decrease. At the same time, the "lost" energy (or most of it) is added to the empty container as kinetic energy - the ultimate slingshot effect.
The result is that the empty container recoils outward at enormous velocity. The increased spacing of the small square blocks in the graphic indicate the relatively large outgoing speed. For a suitable ratio of garbage to container mass, the outgoing speed can be a large fraction of the local speed of light! This sort of thing is observed in nature when a star (one part of a binary system) is flung out of the center of a galaxy at a speed approaching the speed of light.
At the city's power station (top-center), the high-speed container is caught in a suitable water wheel-like contraption that dumps most of the empty container's huge kinetic energy into the rotation of a set of flywheels. These flywheels power generators that feed our city's electricity grid. The empty container is then reused for another load of garbage.
As stated in the authoritative MTW and mathematically shown by Penrose, if optimally executed, the net energy gain of every container's round trip approaches the total rest energy (E0=mc2) of the garbage ejected, plus some fraction of the considerable angular momentum of the rotating black hole. This fantastic power station can produce around one gigawatt-hour of electric energy for every metric ton of garbage dumped into it - GIGO! What a pleasure!
Think about the benefits:
(a) Garbage completely out of sight and smell, hidden by an event horizon.
(b) Nearly 100% of the mass-energy of the garbage is turned into usable electricity.
(c) Some of the black hole's spin energy is thrown into the mix as well.
If this does not qualify for "over-unity", I don't know what will!
Oh, I can already hear the environmentalists of our city raising at least three issues:
(i) We are polluting the black hole.
(ii) We are "burning up" the black hole (if they can figure this one out!)
(iii) Why do citizens have to pay for garbage removal and then (again) for this "free" electricity?
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Regards
Jorrie
Note [1] For the ergosphere, see http://en.wikipedia.org/wiki/Ergosphere