The solar chimney and trombe wall are two simple structures that can be added to any house, and which rely only on the sun and the eart to heat or cool a house. Totally green, and (in the case of the solar chimney) known for millenia, yet we never hear about these..
It's simple: the sun heats the air in a stategically placed chimney, causing the air to expand and flow upward. The pressure in the house is decreased, causing the higher-pressure air to be forced through an undeground tunnel. While underground, the air comes to the temperature of earth, approximately 55F. This not only cools your house, it also creates a nice breeze.
Those of you who are building your houses should keep this, and other passive solar/geothermal designs in mind.