In 2000, Charles Best, a teacher at a Bronx public high school, thought about all the money he and his colleagues were spending on books, art supplies, and other materials for their students. They weren't alone today, teachers still spend an average of $500 out of pocket on their classrooms each year. Charles figured there were people who'd want to help if they could see where their money was going. He founded DonorsChoose, a website where teachers could post requests for classroom resources,...