Chicago's Willis Tower, formerly known as the Sears Tower, was the world's tallest building from 1974 to 1998. While it no longer holds that title, it is seeking to become a green beacon in the Windy City. A Chicago architecture firm has plans to reduce the skyscraper's energy costs by 10 percent using wind turbines and solar panels, and some of the building's owners have reportedly considered painting it silver, an energy-saving idea championed by U.S. Energy Secretary Steven Chu.