The 'Nest' thermostat - brought to you by some former Apple employees; very slick! This device learns your living patterns and automatically adjusts your heating equipment so your home is always warm when you are there, and saving energy when you are not...it even takes into account how your furnace operates most efficiently!
OpenEnergyMonitor - one of the first projects I had in mind at home - real-time energy logging. The only bad thing is that you need to get into your house mains circuit breaker panel. Scary if you don't know what you are doing.