Put simply, the earth has a large magnetic field ranging in intensity between .25 and .65 Gauss. Would it not be possible to use the earth's magnetic field as a stator and electromagnets within a spacecraft to provide orientation torques, or even as a reactionless drive to boost/lower its orbit?
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1Reactionless drives are strictly science fiction. – Organic Marble Nov 21 '19 at 18:43
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@OrganicMarble - they're only science fiction if they do not push off of a medium. In this case, the earth's magnetic field lines serve as a medium. – Justin Braun Nov 21 '19 at 20:13
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Then it's not reactionless. – Organic Marble Nov 21 '19 at 20:26
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1@JustinBraun If you push on the Earth's magnetic field, that force and torque is transferred to the Earth itself. It's a minuscule effect on the Earth because of the huge mass difference, but it's nonetheless a reaction. So far as we know momentum (linear and angular) is always conserved, thus the "Reactionless drives are strictly science fiction" comment. See also Are we changing Earth's rotation? – uhoh Nov 22 '19 at 00:03
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This answer responds to the torque part of the question.
"Magnetorquers" have been used on satellites for decades, mainly comprising a coil of wire for each axis of interest. They have the benefits you mention of being relatively inexpensive and propellant free though will need some careful planning to get the system sized and working well.
They tend not to be used (as far as I am aware) in high precision applications or where the satellite payload instruments might be affected by induction from the current switching on and off.
Puffin
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2Hubble uses them to unload the reaction wheels. https://space.stackexchange.com/questions/36899/does-hubble-need-to-dump-momentum-of-its-reaction-wheels – Organic Marble Nov 21 '19 at 18:45