Space craft that need to fly in formation could use superconducting magnets rather than using reaction mass. Two groups of researchers are working on technology that could replace old-fashioned thrusters, which use jets of gas and Newton's Third Law.
This video shows how spacecraft with electromagnets can fly in formation.
(Spacecraft fly in formation with electromagnets)
The problem with using thrusters is that the reaction mass is ejected to provide the effect; spacecraft can carry only a limited amount of material into orbit. Once the reaction mass is gone, the craft is left without a means of maneuvering.
David Miller of MIT's Space Systems Laboratory is testing the idea using devices that float on a glass plate by blowing air down onto it to simulate the weightlessness of space. Watch this demonstration video.
(Lab demonstration shows how magnetism can orient spacecraft)
In spacecraft, arrays of solar cells would provide power for electromagnets.
This method is not without problems; strong magnetic fields could disrupt delicate electronic equipment. However, shielding can be used in most cases; also, small secondary electromagnets could be used to cancel the main field in small areas.
This technique could run into problems in Earth orbit; the Earth's powerful magnetic field would pull on the magnets, causing spacecraft to begin spinning. However, a group led by Shin-ichiro Sakai of the Japan Aerospace Exploration Agency (JAXA) thinks this problem could be solved by switching the polarity of the magnets several times per minute, achieving a delicate balance.
(The original rag tag fleet from Battlestar Galactica's first season)
We all remember how fleets of spacecraft are shown "parked" in formation; this technology could provide a way for spacecraft to fly in formation without expending fuel needed for maneuvering.