The prospect of reactionless drives has always piqued my interest.
It seems there are two technologies that may be close to verification which would be a huge boon for interstellar propulsion technology.

The first is the EmDrive:

From wiki: "The device is a magnetron with a specially-shaped, fully-enclosed tapering resonator cavity whose area is greater at one end. The inventor claims that the device generates thrust even though no detectable energy leaves the device. The inventor proposes to use it as a spacecraft propulsion system that uses no fuel (other than electricity), and no reaction mass."

EmDrive test prototype:

More info here:

The second is the Mach effect:

From wiki: "The Woodward effect is a hypothesis proposed by James F. Woodward, a physicist at California State University, Fullerton, that energy-storing ions experience transient mass fluctuations when accelerated. The hypothesis is considered to be without merit by respected theoreticians. However, if these doubts are unfounded it is conceivable that the Woodward effect could be exploited in spacecraft engines that would not have to expel matter to accelerate. Several groups of researchers have therefore conducted experiments designed to prove or disprove the existence of the effect."

More info here: