[jmyeet]

Energy is the ultimate limiting factor in everything we do. Cheaper and more abundant energy creates applications just like increasing bandwidth leads to new applications (eg video is really predicated on megabit+ connections).

But two things spring to mind:

1. Computing power. AGI is an obvious one. But can you imagine the virtual worlds people could lose themselves in or even live in permanently? and

2. Interstellar travel ie space highways. Curently interstellar travel is effectively predicated fusion power (or something more exotic) because of the reaction mass problem. But another possibility is concentrating sunlight to accelerate vessels out of the Solar System. This is currently the likeliest scenario for reaching a high percentage of light speed.