Here is an interesting application that allows you to accurately sense the position of a standard Hobby Servo without modifying the servo.
Note: This could also be accomplished with a BS1,BS2, SX, etc.
With a simple modification to the schematic... using PNP transistor acting as a switch across the 470 Ohm resistor, you can also "drive"
the servo you are sensing.
One application might be for a robot to "learn" or record a sequence of movements simply by positioning the servo where you want it to be,
so at a later time your robot would replay the "learned" motion.