The VACTROL, a true analog variable resistance
So, I learned something today. If you need a finely variable resistor with extremely low analog distortion (for say, your high-grade electronic music synthesizer) they make opto-couplers based on a light-dependent resistor (LDR). Current through an LED shines light onto the LDR, which results in a true analog variable resistor. But the cool thing is that the resistance can be easily controlled by other electronics, instead of by some knob the user has to twiddle. Useful, and a bit tricky to do well any other way. Particularly if very analog-y things like THD actually matter. A very handy building block for making true-analog (stepless) AGCs.
http://en.wikipedia.org/wiki/Resistive_opto-isolator