For the serious PC enthusiasts and overclocking crowd who are concerned about the internal temperature of the rig due to extra GPU cards, serious power supplies and so on - this project is for you. Simple, yet quite useful - measuring the internal temperature of the PC with an inexpensive temperature sensor and displaying the results with some coloured LEDs.
Powering the project shouldn't be a problem, as most motherboards should have a spare USB line, or you could split a drive power supply cable and take 5V from that. One could also run a USB cable from the Arduino board back to a spare slot at the rear of the case to allow upgrading the Arduino sketch without pulling the case apart. To get started with your own version - consider using a Freetronics LeoStick. Apart from being one of the smallest Arduino-compatibles on the market, it has onboard USB, RGB LED and a piezo for sounds (useful for a high temperature alarm!)
For more infomation and instructions, visit the project page.