Collpits FM Receiver
Even a very small signal fed into an oscillating circuit can cause it to lock to the incoming signal (Injection locking.) This is exploited in this circuit where the oscillation frequency follows that of the incoming signal.

As the oscillation frequency varies and moves away from the resonance frequency of the tuned circuit the amplitude will tend to fall, which in turn cause the supply current of the circuit to change - this results in the audio signal appearing at the headphone. (Slope Demodulation). This is a result of the transistor rectifying the oscillations and altering the bias point of the circuit.

This type of circuit, although simple, has many disadvantages and so is rarely used in serious designs. 

There's a paper about the operation at https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&amp;arnumber=6774035, unfortunately behind a paywall.