ENERGY STAR distinguishes energy efficient products which, although they may cost more to purchase than standard models, will pay you back in lower energy bills within a reasonable amount of time, without a tax credit.
Included in the tax credit of 30% of cost up to $1500, is biomass stoves, heating, ventilating, Air conditioning (HVAC), insulation, roofs (both metal and asphalt), water heaters (not solar), windows and doors. This credit expires December 31, 2010 and must be on an existing home that is your principal residence. New construction and rentals do not qualify.
The tax credit of 30% of cost with no upper limit is on geothermal heat pumps, small wind turbines (residential) and solar energy systems. This credit expires on December 31, 2016 and is on existing homes and new construction qualify on both principal residences and second homes. Rentals do not qualify.
To find out more about these credits and the fuel cells credit, visit www.energystar.gov
Another good resource of information is: http://www.irs.gov/irb/2009-25_IRB/ar10.html