Unless I've missed anything, the only known, proven to actually work, propellantless thrust system are solar sails (and various beam-powered equivalents). Even

Unless I've missed anything, the only known, proven to actually work, propellantless thrust system are solar sails (and various beam-powered equivalents). Even nuclear photonic rockets slowly use up reaction mass. However, ion drives are already fairly mature and perfectly viable for interstellar travel, if you're willing to wait. Sails aren't yet feasible outside the solar system (and are still in their infancy even within), but with enough power production they might be another option in the future.