Solar energy has been powering calculators for years, but it's only recently become more common in homes due to a drop in costs of installing a solar energy system. While powering your home using solar energy still isn't cheap, it's one of the few ways a homeowner can "live off the grid" by storing extra energy in batteries. The other option is to continue a connection to the utility grid, buying the power you need and selling the power back when you produce enough on your own.
Ground mount solar panels make this home for sale in Shingle Springs, CA (pictured below) very energy efficient.
Reuse gray water
The EPA estimates that an average family of four uses 400 gallons of water every day. While not all that water can be reused, gray water, which is water from your sink, laundry, but not contaminated by waste (i.e., not toilet water) can be recycled. While not as common as some other green home features, gray water recycling systems take the water from your morning shower or from yesterday's washing machine cycle and filters it for reuse in your toilet or your garden.
There's green. Then there's really green. Even a home completely "off-the-grid" won't necessarily have a composting toilet, but when installed properly, a composting toilet is the final nod to going green. A composting toilet will break down the waste through natural decomposition -- the only thing needed is the right temperature (about 65 degrees) and enough oxygen. However to keep this environment, some key things need to be monitored, which may be more effort than most people are interested in.
Composting toilet - Flickr user redjar