The Weatherization Assistance Program (WAP) targets low-income families, providing them the means to make their homes more energy efficient. Thus, not only do you save installation costs, but your savings are also long term. WAP is a federally-based program.
You will also find that there are grant opportunities at the state level. Some grants focus on certain energy incentives. For example, the Renewable Energy Equipment Grant of Minnesota provides up to $4,700 funding for solar space heat or biodiesel systems. The aim with programs like this is to provide an enticement to make the move toward renewable resources. You can find out about programs offered in your state at the Database of State Incentives for Renewables and Efficiency.
While not a grant, you can also refinance your mortgage to make improvements necessary to increase your home's energy efficiency. Funding is provided through Energy Efficient Mortgage (EEM). One of the conditions of this program is that you must have an energy audit of your home done by a certified home energy rater.
If you own land which has wetlands or other important ecosystems on it, you may quality for Wildland Fire Management (WFM) funding. This program funds projects for restoration and rehabilitation of ecosystems on adjacent private lands near federally managed sites. The environmental benefit is that through careful management of your land, you contribute to the health of the ecosystem and the plants and wildlife that depend upon them.