It started out in the mid-1700s with the discovery of the five Lagrange points, relatively stable areas in a rotational system with two bodies (e.g., Earth/Moon or Earth/Sun). But the solar system, with its multitude of planets and moons, is much more complex. The forces exerted by gravity are changing all the time, sometimes stronger or weaker, sometimes in one direction then in another.
These dynamic forces can be modeled, and it’s possible to generate a low-energy flight path for a spacecraft, where it could (theoretically) use no fuel but instead be propelled through the solar system by these changing gravitational forces. In the real world, of course, some fuel is necessary, but it’s a minimal amount.
This type of spaceflight would not be suitable for human beings, as the path meanders significantly, taking much, much longer to reach a destination than a direct flight. But the interplanetary superhighway is ideal for certain types of unmanned probes. And kudos to the people who first came up with the idea—this is definitely thinking outside the box.
Thanks to Josh for the Lagrange point link.
Update: Here is some more recent information on this topic.