The Interplanetary Superhighway
is a concept for moving through the solar system using essentially no fuel.
It has been invented by Martin Lo whilst working for NASA; based on the concepts originally created by Henri Poincaré, Edward Lorenz and many others.
The basic idea is that an object moving through the solar system is following a chaotic path as it gets tugged this way and that by the planets and the sun. By using the subtlest possible adjustments to the speed of the vehicle, early enough, it is possible to massively change the trajectory and transport around the solar system..
In particular, the region around the Lagrange points are particularly powerful- trajectories are created which involve passing through from one lagrange point of one planet to the lagrange of another (as they inevitably line up every orbit or so.)
By using computer searches, a huge number of different trajectories can be explored and it is now possible to plot a course through the solar system, almost for free, with far less fuel needed than a Hohmann transfer.
Of course there are some limitations. For one thing, the position of the vehicle cannot be known precisely, and the mass of the planets isn't entirely known either, so some corrective rocket burns are going to be needed. For another, the kinds of courses that this technique tends to give- they're anything but straight and can bounce around in the earth-moon system for months before leaving for mars. Finally, the launch windows can be very small.
Still, the technique is interesting, and has already been applied on NASAs Genesis project as a way to get to the earths trojan points and back with hardly any use of fuel.