We had a short power outage, and when the power resumed, the fan in our iMac (intel core duo) was stuck on full speed. It sounded like a jet plane. I waited a minute or two for it to slow down, but it didn't stop. Even rebooting didn't affect it.

After some Internet searching, I found that there is a chip which controls the fans, which can get stuck like this. Fortunately it can be reset also.