2011 Top Ten: 7. Cities in developed countries could learn sustainability from so-called slums in the developing world.

Dwellers of "slums," favelas, and ghettos have learned to use and reuse resources and commodities more efficiently than their wealthier counterparts. The neighborhoods are high-density and walkable, mixing commercial and residential areas rather than segregating these functions. In many of these informal cities, participants play a role in communal commercial endeavors such as growing food or raising livestock.

In the future, neighborhoods in the developed world will leverage technology in similar community-business plans, like passive solar energy co-ops, where neighborhoods build their own photovoltaic systems on rooftops, or inner-city biofuel-crop growing perhaps in basement hydroponic gardens. When power generation becomes a community business it’s integrated seamlessly into the area’s future economic development, say designers Pavlina Ilieva and Kuo Pao Lian.