The Blandford Znajek process provides a mechanism by which energy can be extracted from a rotating black hole. This is also one of the best explanations for the energy source that powers quasars

Most of the matter that is in the accretion disk of a black hole will have magnetic field lines threaded through it, as the matter falls into the black hole, these field lines are carried along with it, getting more and more tightly packed the nearer they get to the hole. With a rotating hole, space itself gets dragged around, like water in a whirlpool, these field lines will also get dragged around, becoming coiled and twisted like a rope.

Roger Blandford and Roman Znajek (a gradutate student) studied this system at Cambridge, England. Their calculations gave some quite surprising results. You can treat the the whole arrangement of hole, field lines and hot plasma from the infalling matter, as an electrical circuit.
Electric currents flow along the field lines into the poles of the hole, over the 'surface' of the event horizon, and out of the horizon near the equator. At a distance from the hole, this current then dumps it's energy into the plasma, which is blown away from the hole in the form of jets from the poles. This makes the hole the voltage generator, the magnetic field lines the wires, and the plasma the load of the circuit!
Of course electric charges cannot really flow across the mathematical object that is the event horizon, this description is only useful if you want to understand how magnetism, electricity and plasma behave outside a black hole.

As a black hole can have 29% of it's mass in the form of rotational energy, this process could serve as a harnessable energy source. See E=mc2 and note that these black holes in quasars are super-massive black holes, with perhaps a mass of 2 x 1041 grams
Take a spinning black hole, build a giant super-conducting ring at some distance around it.(So that the ring is pole to pole, rather than around the equator). Then thread the black hole with magnetic field lines; the superconducting ring can generate forces to prevent these becoming un-threaded. The field lines can act as wires, carrying positrons inwards to the poles, and electrons inwards towards the equator. (All directions are in when you're talking about black holes, but using electrons and their anti-particle twins positrons makes this work). The current flow is from the equator to the ring, where power is extracted, then back to the poles of the hole, where energy is (ultimately) extracted from the rotation and fed back to the ring.
Eventually this process will slow down the rotation speed of the black hole, but as the energy content may be thousands of times greater than our sun (depending on the mass of the hole and it's rotation speed), it could still provide power for millions of years....

Primary source:- Black Holes and Timewarps by Kip S. Thorne.