Matter-energy conversion, both ways. Star Trek replicators are energy to matter only, although you never find out what happens to the containers created by them (e.g. the cups, dishes, etc).
That way, all you will ever need is sufficient matter of any kind to replace the energy you use, and that will be hardly anything. 1 Kg of matter is 9x10^16 joules, or about 2.5 times the annual output of a major power station, or about 7.5% of the entire UK electricity usage for a year.
Pollution could be ended - just convert the pollution to something else at source.
Deforestation could be ended - what would be the point of it?
Everyone would have endless fresh water - just convert anything else to fresh water.
No-one would starve - convert dirt and rocks to fine food.
Electric vehicles would become very practical - a pinch of matter would provide maximum power for days. Although you could also use an ICE for the glorious noise of it - convert the heat and the exhaust back into petrol and you'd still only need a pinch of extra matter.
And, because it's a cool idea, you could configure a unit in your toilet to turn your pee into gold and your poo into diamonds. It would even be practical to store it all as gold blocks, as the density and lack of reactivity of gold would make it well suited for storing matter until you wanted to convert it to energy or other matter (e.g. food).
Of course, the entire world economy immediately collapses as both agriculture and manufacturing cease to be of any use.
