The phrase “side hustle” seems to have become a bit of a buzzword lately, as the gig economy has made it easier for us to adopt multiple money-making strategies alongside the 9-5. Yet in some cases, a simple side project that started out as a way to make extra cash can turn into an extremely successful business. Click or scroll through to see the 15 most impressive side hustles that turned into multimillion- and even billion-dollar companies.