Last year NASA successfully tested a concept electromagnetic drive that derived its propulsion from bouncing microwaves inside a closed container--in effect, a reactionless drive. However, skeptics indicated that the results showing thrust could have been due to environmental factors since the device was not tested in a vacuum. But io9 reports that recent tests performed in a vacuum confirm the tests from last year.
Update (May 2, 2105): The Chicago Boyz explain in greater detail the significance of the EM drive, if it works. Especially when considered together with Lockheed's Skunkworks planned development of a compact fusion reactor. It opens up the solar system, at the least.