Everyone agrees nuclear fusion is the most promising energy source out there - IF we can get away from the "promising" part and make it work.
In this article, Michael Schirber gives a good overview of two approaches to confined fusion reactions which will be demonstrated in the next couple of years. One is ignition of fuel pellets by lasers, which should be self-confining and works a little like your car engine, with a series of bursts of energy. The other is magnetic confinement, heating a hydrogen-gas fuel to stupendous temperatures and confining the continual reaction that results with magnetic fields. Both cost billions of dollars to test - no other energy source has as much up-front cost as fusion. But none has as much potential, either.