Spur gears <link> are near ubiquitous. This is partly because the electric motor is a near ubiquitous device but spins inconveniently fast (10,000-100,000 rpm) for moving things on the human scale - 100rpm would be the ballpark for a bicycle pedaling cadence for example. So, we put a small spur gear,
often called a pinion, on the motor and a larger spur gear on the thing we want spinning. This is an excellent arrangement because it's simple, robust and >95-99% efficient.
I have such pinion/spur gears in an RC car. Annoyingly, 1:16-1:8th scale cars are noisier than full sized electric cars. There are many reasons for this, but perhaps the main factor is putting power through small gears means fewer teeth are in contact and so teeth have to be relatively large. Plus the transmissions are subject to shocks such as landing from a not-scale 2m/6' up at full/zero throttle. This means that teeth are not in contact all the time. You can hear this when accelerating and then backing off throttle. You hear a kind of chattering sound. This is because the gear train is oscillating, between the motor slowing the car and the car slowing the motor. Each tooth is alternately contacting the one in front and behind.
Belt drives are a viable alternative. They are quiet. The belts stretch a little absorbing speed mismatches. However, they're not as efficient, require more maintenance and dictate the layout of the car to a greater extent. Magnetic drives and even gearing also exist <link>. While elegant, they lack torque density - where spur gears excel.
Can we combine the two? Let's take our spur gear and make it so that each tooth has a North magnetic pole and a South magnetic pole arranged circumferentially, so that the working face of each tooth is either North or South in such a way that all teeth facing clockwise are North, say and all facing anti-clockwise South. Now, our motor pinion, is set up in the opposite polarity. When the gears are meshed, each pinion tooth will sit between two spur teeth in such a way as it's North Face faces the Spur's North face (and South faces South). Now the Pinion and Spur teeth will "float" at the maximum distance from each other.
Now, I'm not proposing that this will be a contactless system of torque transfer (this is baked), the magnets don't generate nearly enough force. As the motor spins up it will push the teeth into contact with one another and transfer the force in the conventional way - preserving the torque density we selected spur gears for in the first place. But, SOME force will be transferred before teeth contact, this reduces shock loading - acting a like a magnetic cush-drive <link>. Crucially this force acts against torque transfer in both directions, acting against the oscillations I hear in my RC car gear train.
Problems? Well, making such gears is tricky. Large steel gears could use a jig with electromagnets between each tooth, energize the coils as the steel is cooling through it's Curie point. Would have to accommodate for heat treatment. Small rare-earth magnets could be included in the injection-molding process of plastic, usually, nylon gears. Although this would be fiddly. Any ferrous metal chips from wear in the transmission would end up in the worst place, on the gear tooth faces.