Netduino based self regulating water heater
The idea that I came up with was to design a scheduler, configured to follow our weekly hot water usage pattern and capable of lowering the water heater temperature down to a minimum during off-hours.
I wanted it to be cheap to build with easy-to-find parts and very reliable as my wife does not appreciate cold showers: I decided to use a netduino-mini micro-controller, an AdaFruit DS1307 real-time clock and a servo to adjust the temperature of the water heater.
More: http://fabienroyer.wordpress.com/2011/02/22/saving-energy-with-a-netduino/