Perhaps no two companies have helped to reshape the retail industry more than Wal-Mart Stores (NYSE: WMT) and Amazon.com (NASDAQ: AMZN). With its nearly $500 billion in annual revenue, Wal-Mart tops the Fortune Global 500 list for the world's largest corporations by revenue. And after two decades of torrid growth, Amazon is now the dominant online retailer.As these companies have risen to power, they've earned a fortune for their investors along the way.