6. Cambridge, Mass.
In 2008, Prevention Magazine named Cambridge "the best walking city." Thoreau's Walden Pond can be found in nearby Concord, and education powerhouses Massachusetts Institute of Technology and Harvard University are located here. In 2002, city officials implemented a major climate protection plan and today most city vehicles are fueled by B20 biodiesel or electricity. All new construction or major renovations must meet LEED standards. And a project called "Compost that Stuff" collects and processes organic waste from residents, restaurants, bars and hotels.