I've read a few articles about Earth-Mars Cyclers, an idea that is perhaps most associated with Dr. Buzz Aldrin, although he was not the first to come up with the general concept. The idea is to discover or design special orbits around the sun that intercept both the Earth and Mars on a regular schedule. A large space-station like spacecraft placed on such an orbit could provide regular "shuttle service" between the vicinity of Earth and the vicinity of Mars at very low continuing cost (of course the cost of building the cyclers and establishing them in their orbits would not be low - it's a long-term-presence approach to Mars access).
It turns out there are many such orbits, and there are various problems and trade-offs associated with their periods, the energy needed for "taxi" spacecraft to rendezvous with them as they pass by Earth or Mars, and other factors. In order to have a reasonable transit time in both directions, you would need to have two or more cyclers in similar but complementary orbits.
Damn Interesting did a nice story on Mars cyclers, and this 2005 article by Buzz himself is a very good intro. This is a fascinating concept for building some key infrastructure for a spacefaring civilization that extends beyond the "local neighborhood" of the Earth and Moon. I would like to experiment with these orbits in Orbiter if I could find the time.