The fiscal cliff bill extended several key energy efficiency provisions, making several which had expired in 2011 retroactive to Jan. 1, 2012 and lasting through Dec 31, 2013. For consumers, they key provision is a $500 tax credit for energy efficient upgrades like windows or HVAC systems. Note that if you claimed the credit previously, you cannot claim it again. Please check IRS guidance when it comes out to be sure you can claim the credit.
More info available here from Consumer Reports, including a buyer’s guide for energy efficient products. There is also a great explanation of all the efficiency provisions from the Energy Efficiency Business Coalition.