[usrusr]

On an unfortunately very theoretical (but apparently already patend-encumbered, from what I can glean from Wikipedia) level there is also the concept of air-breathing electrical propulsion, an ion engine replenished from the the same trace atmosphere that causes the drag the engine is supposed to counter. Basically a solid state propeller that can still work in very thin air.

From my layman's understanding, because the effect of aerodynamic shaping breaks down at very low pressure, exhaust speed would have to be travel speed (TAS) x (total cross section / intake cross section) to keep orbit. Assessing whether that puts the concept in the realm of feasible technology or not is beyond my skills, but at least there seem to be projects working on that question. If it does work out, it would completely change the economics of LEO use.