Marc de Vinck has been producing an excellent series of tutorial videos that show you how to make things with the Arduino microcontroller. In this third installment, he uses the Arduino to control a servo motor with a potentiometer.
If you're interested in learning how to use the Arduino, I recommend starting with the slim, easy-to-grok book, Getting Started with Arduino, by Massimo Banzi, the co-founder of Arduino.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. His new book is Maker Dad: Lunch Box Guitars, Antigravity Jars, and 22 Other Incredibly Cool Father-Daughter DIY Projects