Formation flying in space, without propellants.
Electromagnetic formation flight (EMFF) gets around this propellant problem by turning the satellites in a formation into electromagnets. By using a combination of magnets and reaction wheels, spacecraft in formation can move and change their attitude and even spin without propellant. Satellites can change their polarity to attract or repel one another, turn, or shift their relative positions in any manner that doesn’t require changing the center of gravity for the entire formation.
A prototype is going to be tested inside ISS in the near future.