Jeremy S. Cook designed a tiny PCB to cool the Pi:
In a previous post, I did a very brief introduction to the world of Bash scripting in the context of Raspberry Pi single-board computers. It’s an amazingly powerful tool, capable of administrative tasks like batch file renaming, making decisions, and more. While this scripting interface is available for any Linux system, the Raspberry Pi’s GPIO pins make it even more powerful, allowing it to control physical devices, like an LED directly, or even motors and other higher current devices indirectly via a transistor.
As it just so happens, the Raspberry Pi doesn’t come with any sort of active or even passive, cooling solution, and it’s pretty common to simply hook up a fan to run at all times to its 5V power supply. This seems to work fine, but when I noticed the Pi that runs my 3D printer (in a hot Florida garage) was overheating, running it all the time seemed a little silly. After all, power is applied to the Pi constantly, but it’s actually used on a very intermittent basis when I’m printing something.
(Script and PCB design available on GitHub)