I'm a firm believer in maintaining a portfolio that's mostly full of rock-solid stocks that you don't need to continually worry about. Having said that, however, there's nothing wrong with taking some small portion of your capital to speculate on some younger companies with tremendous growth potential.If you have a long investment time horizon -- a minimum of five years or so -- and the stomach to withstand some growing pains along the way, here are three companies that have the potential to produce some pretty impressive growth over the long run.Image source: Getty Images.Continue reading