Tc-06
Nick made a temperature-based computer fan controller that use a reverse-biased diode for temperature sensing –

From a design standpoint, the only complaint I have about my Shuttle small form factor desktop is the extremely fragile on-board fan controllers. All three have died over the past two years after being used to regulate the single on-board fan with respect to the system temperature. The last one gave up the ghost a few days ago as my machine suddenly shut down and a red light came on inside.

Figuring that I should just go build a more substantial fan controller myself, I promptly went to the lab to look for a thermistor. Unable to find one, I settled for the next best thing: a diode.

μblog: engineering from the trenches » How to make a temperature-based fan controller without a thermistor – Link.