Google is allowing their employees to work on this project for 20% of their work time. Their projects may become part of the larger project that is currently available in the San Francisco Bay Area, Fresno (California), and the Boston area.
Project Sunroof looks very similar to the application by Google call Google Earth. It uses aerial mapping to show your roof and the solar energy potential.
Since this is currently only a 20% project, it is limited and not all places are available on the map yet. However, the creator, Carl Elkin, notes that they wish to explore being able to make this a tool that everyone can use, just like Google Earth.