My Apple Time Capsule runs hot - approaching 50°C to the touch - this isn't helped by the lack of space it has and the Apple TV stacked on top (I've added some legs to give some space below and above for air to flow. On large backups/restores during the summer - the device often went into over heat warning mode. There's also worrying reports of the power supply overheating and Time Capsules dying.
I originally thought of a standard PC fan with some kind of thermistor /thermocouple/sensor and a simple control circuit to activate the fan when temperature gets too hot, but stumbled across Arctic Cooling Pro TC F9 fan which has a temperature sensor on a 40cm cable to turn up the fan speed when the temperature rises above 32°C.
Replacing the plug with a 2.1mm power jack to fit a 12V wall wart PSU and creating a stand with a loop of wire coat hanger sleeved in heat shrink to give it a nice black cover to match the fans case and the unit was ready to go. Taping the end of the temperature sensor to the top of what felt the hottest part of the Time Capsule (over the power supply next to the power inlet) reduced the temperature to less than 30Â°C. I though the fan might be too low powered initially but it manages to cool all the devices on the shelf it's on.
The unit is silent - at least quieter than the ambient noise and hopefully will extend the life of the Time Capsule.