Woodward / Mach Effect Reactionless Drives
This is really remarkable. Based on the principle of the Higgs field (that mass is not constant) what might seem like an impossible Rube-Goldberg machine is, in fact, a real possibility for thrust without the messy rocket exhaust. You can read Mr. Woodward's ideas in great detail in his book "Making Starships and Stargates" from Springer publishing.
The video below shows a scale model example of what is being talked about. If mass fluctuates, you can push in one direction when the mass is low (increasing velocity cheaply). Then the system can "relax" when mass is high (not decreasing velocity very much). A small net velocity remains. Repeat, slowly adding velocity by increments.