You will find HDD Fan Control in your downloads folder.
It's both the demo and the full version (with license):
You can try it out for 1 hour after installation
then you must purchase a license to unlock the full version.
Instructions and Help for HDD Fan Control can be found in Support.
Reduce Fan Noise and Protect your Drive.
The great blog TUAW has published an entry about HDD Fan Control and it solving the issue of replacing the hard drive in an iMac, the article can be read here HDD Fan Control software addresses iMac hard drive replacement issue