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.
The Potentiometer Board is an effective and low-cost solution for testing analog inputs on the prototype board.