Measuring Inductance
I once needed to get an idea of the effective inductance of the primary coil of a custom rotary transformer, so I could get an estimate of its magnetizing current.

Since I was driving it with a square wave anyway, I just put a resistor in series with it to sense the current, and measured the slope of the current waveform on an oscilloscope.

$$V(t) = L\frac{dI(t)}{dt}$$

Since V(t) is constant in this case, I(t) is a linear ramp. Solving for L is straightforward:

$$L = V\frac{\Delta t}{\Delta I}$$