There are many ways to save energy producing hot water.   The obvious one is solar hot water.  Solar hot water systems have to be carefully planned such that they can dissipate excessive hot water.  Overheat is a common cause of failure.

But the method that returns the greatest heat for the buck is a heat pump hot water heater:

The Nyletherm-1

This device will cut your hot water bill by 50%.  Note that it is pulling the heat from the surrounding air.  In a hot climate one could use the cold air produced to help cool the home, providing virtually free hot water.  It is a simple installation and can be used in conjunction with the existing hot water tank.

Also, don't fall for the gas fired instant hot water heater.  They actually waste energy compared to an ordinary electric hot water heater.  Of they are placed near the point of use, they can same energy.  But then a small 2-5 gallon electric hot water heater can be placed there as well and you won't have all that energy going up the flu.

One of the biggest energy wasters is the hot water recirculator.  The pipes are always poorly insulated, and can consume a huge amount of energy.

Our hot water system has a 2.5 gallon tank in the master bath.  This gives us instant hot water without the wasted heat in long runs of pipe.  The shower is fed from an ordinary 50 gallon resistive hot water heater.  This hot water tank is fed from pre-heated water derived from our geoexchange desuperheater, which provides most of our hot water during the winter.

A recirc pump can be switched on from any sink.  The recirculator can be turned on 5 minutes before a shower to avoid wasting hot water.