In search of the next big thing, we look at the Wall Street Journal’s Top-50 American start-ups. Economies change. Little start-ups become great pillars. This week, Google became more valuable than Microsoft and Wal-Mart. Google didn’t exist fifteen years ago. So what’s the next big thing? The thing that may drive our next economy? The Wall Street Journal is out with a top-50 list of the nation’s hottest start-up companies. Read it here.