Beginning on January 1, 2011, there is a $500 lifetime limit on the tax credit. That means if you've gotten any energy efficiency tax credits from 2006 - 2010, then that counts towards your lifetime limit, and you can't go over $500. For example, if you got $800 (of then $1,500 tax credit) in 2009, then you are not eligible for anything more in 2011. If you got a $400 tax credit in 2006, then you can get an additional tax credit of up to $100 in 2011. The $500 tax credit can be on a single, or multiple products - but not more than $500 total for your lifetime.
Many of the energy efficiency tax credits are limited to 1,500 combined total for 2009 and 2010. The 1,500 maximum tax credit covers insulation, windows & doors, roofs, HVAC, biomass stoves, and non-solar water heaters.
Improvements not affected by the 1,500 cap (and in effect through 2016) include:
You can get both the 30% tax credit (up to 1,500) and the 30% tax credit (with no upper limit). For example, you could replace your windows and get 1,500, and purchase a new geothermal heat pump and get an additional 30% tax credit (with no upper limit). However, these are non-refundable tax credits, so you can't get more back in tax credits than you pay in federal income tax
Even if you received the full 500 for the tax credits back in 2006 or 2007, you are still eligible for the full 1,500 in 2009 and 2010.