It monitors and regulates the heat in a aquarium, using the dSwitch from Dexter Industries:
"The NXT is programmed to monitor the temperature using the LEGO temperature sensor and the dSwitch NXT-G block is used to turn the light on and off based on temperature thresholds. My current setup has the dSwitch turn the light on when the water temperature is below 72F and turn it off when it hits 79F. With NXT-G I am also able to control how often it polls the temperature to ensure that the granularity of monitoring is over a longer period of time."Interesting application!