Do you have a few minutes? Feel like seeing how energy efficient your home is? Go grab a bunch of your old utility bills and sit down with EERE's Energy Calculators & Software for Homes. There's a lot to search through, but some of these calculators are pretty fun to play around with.
This page has calculators for just about everything a homeowner would be interested in. There are tools here for home-wide energy efficiency, insulation, appliances, and water usage. There are also whole sections on renewable energy systems, space heating and cooling, and water heating. If you want to know how much you're spending on something and how much you should be spending, you can pull out a lot of useful information from these.
There are also a few calculators for vehicles that let you see how much money you'd save with a more fuel-efficient vehicle. I particularly like the one that compares your car's performance with a hybrid vehicle. Which is fun, if not a bit tragic, because there is something horrifying about seeing how my poor little one-year-old car will use $3,000 more in gasoline during its lifetime than its hybrid equivalent. Oh! And it'll also spew 400 more pounds of "Carbon Monoxide (Poisonous Gas)" into the air. Isn't that charming!
In any case, the EERE Calculators are a fun project for a rainy (or, considering the season, snowy) day. Some of the calculators are more technical than others, so don't be afraid to hunt around. And unless you're an energy professional or a home builder, you can probably steer clear of the software.
Elizabeth Spencer is a communicator at the National Renewable Energy Laboratory, which assists EERE in providing technical content for many of its Web sites.