Recently there has been discussion of the near term future of a space rated VASIMR plasma rocket which can be scaled up to several megawatts to enable 39 day (one way) trips to Mars. This site has looked at combining a small nuclear reactor that has a commercialization target of 2013 with the VASIMR which is also expected to be tested in space in 2013. Both would need a few more years of work and production but a combined nuclear-VASIMR is technically possible before 2020.
Nextbigfuture reader Tom Craver had the idea of using VASIMR to accelerate and decelerate a Earth-Mars Transit Habitat:
VASIMR could accelerate a transit hab and tanks of rocket fuel on an orbit that swings back past Earth. A light and fast crew shuttle (too cramped for long trips) would rendezvous. At Mars the cramped crew shuttle would be re-fueled and the crew would decelerate into orbit. This should cut transit time about in half again (about 20 days) because it would be mostly up to speed by the time the crew boarded, and the ship wouldn't slow down nearly as much as it approaches Mars.
UPDATE: Will need to look at this more closely based on orbits, as well as acceleration to see how much lower speed travel is reduced with a relay system of vehicles. A powerful VASIMR has a pretty strong low gear acceleration, so there may not be as much benefit from this relay transfer system.
There was a (65 page pdf) 1994 NASA study "Ion engine propelled Earth-Mars cycler with nuclear thermal propelled transfer vehicle, volume 2" by Meyer, Rudolf X.; Baker, Myles; Melko, Joseph However, the study was looking at far weaker ion engines so the calculated times were slower than a straight up VASIMR.
The goal of this project was to perform a preliminary design of a long term, reusable transportation system between earth and Mars which would be capable of providing both artificial gravity and shelter from solar flare radiation. The heart of this system was assumed to be a Cycler spacecraft propelled by an ion propulsion system. The crew transfer vehicle was designed to be propelled by a nuclear-thermal propulsion system. Several Mars transportation system architectures and their associated space vehicles were designed.
There was an eight page article on the various near term ways to from Earth to Mars.
Buzz Aldrin has been advocating an Earth Mars Cycler that takes 5 months for each one way trip between Earth to Mars. Tom is suggesting speeding up the cycler with plasma rockets (and also using plasma rockets for decelerating for transfers) and not just depending on gravity.
A cycler trajectory is a special kind of spacecraft trajectory that encounters two or more bodies on a regular basis. Cyclers are potentially useful for transporting people or materials between those bodies using little or no propellant. Instead, they rely on gravity assist maneuvers to keep them going.
Once we have nuclear powered VASIMR spaceships, we can pre-deliver well shielded (more massive) habitats to human destinations, such as Mars orbit. So we don't need to take the risk and expense of going to Mars surface just for exploration - we can stay in orbit and use remote presence robots to "virtually" visit and explore Mars surface.
Analysis of Various Two Synodic Period Earth-Mars Cycler Trajectories (8 page pdf)
A Megawatt class VASIMR engine cluster (five 200 KW VASIMR) for 2013