No more heavy fuel, perilous reactor-core explosions, or dilithium crystal shortages. The Casimir Effect could make fuel-less space travel possible.
To understand the Casimir Effect, we must first establish that the unforgiving nothingness of space - which has claimed the lives of so many minor characters in science fiction films, and the sanity of the Reavers in the Firefly TV series - is not truly nothingness at all. It is filled at all times with irregular electromagnetic waves, or by spontaneously appearing and disappearing particles, depending on what side of the particle-wave duality you come down on at the moment.
I'll let Scientific American explain it in terms of waves, and I'll give a run down on the particle side of things. Imagine you are trying to get to someone through a large crowd of randomly moving people. (Feel free to apply a dramatic purpose to this scene and outfit the crowd in period costume.) At first it will take effort to move through the crowd of people. Then, as you get close to each other, there won't be any more people to fight through. There will be a lot of people around you, though, jostling you towards each other. The force of the crowd around the two of you will push you into each other.
That is what the Casimir Effect does to two metal plates held close enough together. As the plates get closer to each other, there will be more particles pushing in on the two plates than there will be pushing out between them. Eventually, they will be pushed towards each other and stick together.
For a long time the Casimir Effect was less a fascinating aspect of our universe than a pain in the ass for nanotechnologists, since it meant that they had to deal with quantum mechanics sticking their materials together as they tried to build the tiny mind-controlling robots we all know they want to make.
It wasn't until physicists came up with a way to reverse the Casimir Effect that things got interesting. Under the right conditions, the two plates will push away from each other, instead of towards each other. It made frictionless technology possible. It also made the Casimir Effect more versatile.
While both effects occur only under very specific conditions, and while they are very slight, they are examples of the supposed 'vacuum' of space creating energy. This is a source of energy that would never run dry, and would be available anywhere. If there were a way to use the push and pull between the plates to propel a starship forward, that ship could travel forever without fuel, harnessing energy from space itself.
At the moment, NASA is "doubtful this energy can be tapped." The energy is there, however, and its potential remains.