As a source of renewable energy, consider the homeless person, addicted to drugs or alcohol, pedaling out toxins on an exercise machine attached to a generator. Dr. Sam Slishman has. The emergency room physician was standing under the Eiffel Tower a few years ago, annoyed with the trinket sellers. What a waste, he thought. "Every city has their trinket sellers. These trinkets don't contribute anything," he said. "I'd rather pay them to sit on an exerciser and generate electricity." The idea stuck with him and became part of Endorphin Power Co., his solution for several social issues at once: It...