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.