An adjustable potentiometer can open up many interesting user interfaces. Turn the pot and the resistance changes. Connect VCC to an outer pin, GND to the other, and the center pin will have a voltage that varies from 0 to VCC depending on the rotation of the pot. Hook the center pin to an ADC on a microcontroller and get a variable input from the user!
This pot has a 7mm mounting diameter and is breadboard friendly. Check the datasheet for dimensional drawings.
Look below for the plastic knob that goes with this potentiometer.