It was seven years ago that the market saw its first 1 trillion-dollar company, when Apple (AAPL 0.54%) crossed that threshold. Since then, many more have reached trillion-dollar valuations. And according to some analysts, it may not be long before there is a stock that's worth $5 trillion.Microsoft (MSFT 1.33%) and Nvidia (NVDA 1.81%) are among the most highly valued stocks in the world today. They are trading at record levels and their valuations aren't exactly modest. And yet, analysts are projecting muc ...