The terms "green" and "energy-efficient" are all over the place, and home features are no different. You can start out making a house more environmentally friendly when you're building it by working with an architect and designer who specializes in such things. It might even include how you situate the house in relation to the rising and setting of the sun and the placement of windows and doors (which influences cooling and heating costs). But if you didn't start out with that in mind, there are still lots of different green and energy-efficient features that you can use in an existing house. Not only can some of them save you money over time, but having a house with green features can be a big selling point for an eco-conscious buyer.
There are several different directions you can go in; energy efficiency, air quality, water conservation and energy generation are just a few. For example, making sure that all of your major appliances have Energy Star labels and you have a high-efficiency water heater and HVAC system make a huge difference. An updated HVAC system with good filtration also affects air quality, but so does making sure that your home is painted with low-VOC paint and the flooring you choose is low on pollutants and renewable (such as bamboo). Installing double-paned windows will keep in heat in the winter and your air conditioning in the summer.
Ready to take it to the next level? Look into water conservation. You can install gray water systems, which use water that isn't drinkable (but isn't sewage, either) to do things like flush your toilets or water your lawn. Or just collect rainwater. Solar panels are just one way to generate electricity that you can actually use in your home -- you can also have wind turbines (or windmills). Yes, some of these features can get pricey, but if being green is your thing, it might be worth it to you.