Our buds at uC Hobby have another winnerin their Hobby Parts for Articles program:
Oscar Gonzalez gives us a simple tutorial for speed controlling a PC FAN and reading a LM35 temperature sensor with an Arduino microcontroller. He covers the use of an optocoupler, PWM (Pulse Width Modulation) and reading analog voltages with the Arduino. He even has a video showing the fan speed control in action.
Source:http://www.uchobby.com/index.php/2007/09/23/arduino-temperature-controlled-pc-fan