Facebook long relied on a programming language called PHP that's simple to learn and use, but devours computing time and energy. Haiping Zhao, 40, labored for two years—at first alone, then in a team of three—to build HipHop, which transforms PHP into C++, a much more efficient language. The endeavor required looking at every line of PHP code on the massive site, but the savings are as astonishing as the project's ambition: "We've reduced the CPU usage on our Web servers on average by about 50%," says Zhao. HipHop was released as open source in February "so the Web at large can benefit." While this may seem like geeky science at work, the Princeton grad, who also has a master's from New York University in pharmacology and molecular biology, thinks differently. "Writing a piece of code," he says, "is crafting a piece of art in my mind."