Social entrepreneurs take the best of social activism and business enterprise and stir them together seamlessly. The numbers of new ventures is staggering. Social entrepreneurs—enterprising individuals who apply business practices to solving societal problems, such as pollution, poor nutrition, and poverty—are now 30,000 strong and growing, according to B Lab, a nonprofit organization (my company WorkSavvy is a founder member) that certifies these purpose-driven companies. Together, they represent some $40 billion in revenue. Business Week features 28 of the America’s most promising social entrepreneurs.