The EM Drive is a reactionless drive originally developed by a man named Roger Shawyer. He to use a magnetron to create and bounce microwaves around a chamber. The thrust comes from an imbalance in the resultant forces coming from the reflections due to relativistic effects. The EmDrive has been panned by the scientific community and there has been no independent peer review.
With that said, after the original British and then American interest fell through, the Chinese picked it up. They now claim to have developed an experimental simulation that confirms the validity of Shawyer's drive. The thrust produced is comparable to ion drives but the main differences are that there is no reaction mass and weight. The Em Drive weighs 7 kilos and produces 85 mN (milli-Newtons) of thrust. NASA's NSTAR ion thruster weighs upwards of 30 kilos, uses four times more power, consumes 10 grams of fuels per hour and produces 92 mN of thrust. I have not been able to confirm the weight of the NSTAR but according to space.com, NSTAR uses just over 4 grams of fuel per hour. A bit of an overstatement.
The EmDrive is something that I would need to see to believe until then, its a perpetual motion machine.