The US Dept. of Energy (DOE) and Environmental Protection Agency (EPA) each have online resources describing home energy efficiency measures. DOE's Energy.gov website and EPA's EnergyStar.gov each have an abundance of tips for homeowners to save energy, but with so much content these sites can easily be overwhelming and neither one has a content map. Here's a few areas to focus on if you're interested in saving energy, money, and your finite web surfing time. Each site also has a search box that will provide a list of site links relating to a specific question or topic.
DOE's Energy.gov site has a section devoted to residential energy efficiency in the middle of the main navigation menu under EnergySaverHome, and there are sub-menus for different topics like water heaters or windows/doors. DOE's AskEnergySaver feature seems like the most updated part of the site; each month has a theme, and questions can be tweeted or emailed to be answered in a post near the end of the month.
EPA's EnergyStar.gov site is mostly devoted to residential energy efficiency, though there's a section for businesses as well. The At Home tab in the main navigation menu is probably your best bet unless you're building a new home. The 5 main areas of At Home include guiding or tracking progress with Home Advisor or My Energy Star, and how-to's with Ways to Save and Knowledge Center. With many areas of overlap and some long pages that need a lot of scrolling to get through, it may take a few minutes to get the lay of the land.
In Massachusetts we have access to MassSave home energy audits (through programs like Chelmsford Saves) that not only evaluate the need but also pay for many basic energy efficiency services such as lighting or air sealing. For us the main value of these types of websites may be to help select specific products or appliances that are subject to MassSave rebates.