I have a geothermal HVAC system, and it's not very well designed. If the loop temperatures drop below a certain point it becomes ineffective, and I need to manually switch to a secondary heat source.

In a single afternoon, I was able to hook some one-wire sensors into the loops, attach them to a widget that made the sensor data available via IP, write a daemon that monitors the temperature, and then automatically notifies the IP-enabled thermostat to flip over to secondary heat if the loops go below effective temperatures, and to flip back when they recover.

The next day, I made it so the loop temperatures were recorded and graphed, and that I receive an SMS alert if there's a complete HVAC failure (loops too cold + secondary heat failure).

That was some awesome tinkering.