The Dept. of Energy has a page discussing heat pump water heaters. Heat pump water heaters are similar to heat pumps that heat your home. They use energy to move heat from inside to outside rather than creating heat. Heat pumps save energy because its easier to move heat than create it.
This report on heat pump water heaters examines their cost efficiency. It says:
"Assuming an electricity cost of $0.10/kWh, savings in annual hot water heating would be $262. Assuming an installed cost for an exhaust air heat pump water heater to be $1,500, the simple payback would be $1500/$262 or 5.7 years. If you are replacing a failed water heater, assuming a cost premium of $1,200 for installing an exhaust air HPWH rather than a conventional electric water heater, the simple payback would be 4.6 years."
But on the downside the heat pump water heaters are not very common and may be hard to find. According to the ACEEE they say "Several manufacturers have discontinued their lines of heat pump water heaters due to lack of consumer and contractor awareness". They give a short list of manufacturers none of which are familiar names.
One interesting product is an add on heat pump for a normal hot water tank. The
Airtap product (pictured at right) from AirGenerate is $500 and adds on to a standard hot water heater. They claim that it takes 2.5 times less energy than a normal hot water tank alone. Compared to a standard hot water heater that uses $400 electricity a year the Airtap should use about $160 in energy costs. That equates to a $240 annual savings from a $500 investment for a payback period of about 2 years. The Airtap plugs into normal 110V AC line and their installation demo video claims it can be installed in 1 hour.I found a Review of the Airtap from Environmental Building News which was positive overall.
There is one important 'gotcha' about the Airtap though which is mentioned in the comments of that review. Using an Airtap for a hot water heater that is in a room heated by electric heat is not recommended. The reason being that the Airtap sucks heat out of the surrounding air and if you're heating that air with your furnace then you're paying to create that heat to begin with. The Airtap would cool your interior space and then your furnace would have to kick in to reheat it.
If you are due to replace your hot water heater then investigating heat pump water heaters is a good idea. They don't quite seem common place however so you may need to hunt a little to find one. The Airtap upgrade unit does look like it would be very practical solution at least if your water heater is not in a primary heated area.