By Michael Johnston for ETF DatabaseIn the San Francisco Bay area during the 1970s, a revolution was brewing. The emergence of an area dubbed "Silicon Valley" brought about a surge in technological development that changed the world forever. Two young innovators at the forefront of that revolution, Bill Gates and Paul Allen, founded what eventually became Microsoft ( MSFT). Sensing a tremendous opportunity in the computer market, Gates dropped out of Harvard to pursue his dream -- no doubt a decision that changed the course of tech history. Microsoft, of course, would come to dominate the personal computer market, making its founders billions of dollars in the process. Gates is counted as one of the wealthiest men in the world. So what does a billionaire do with the massive fortune he has accumulated? Invest it, of course. Gates remains active with his fortune, owning several investment firms as well as the being the single largest shareholder in Microsoft. He is the author and co-author of several books and is a well-known philanthropist. Though no investments have paid off as handsomely as his initial bet on Microsoft, Gates has built a rather impressive track record over the past several decades, multiplying his fortune several times over. GATES' BETS Among Gates' recent areas of investment focus: cleaner car technology, specifically through the engine manufacturer EcoMotors. Gates recently allocated $23.5 million to this Detroit-based company because he believes our combustion engines are quickly becoming a thing of the past. The engines EcoMotors hopes to build "will have less mass, will cost less to make, will be more fuel-efficient and will generate lower emissions," Jolie O'Dell writes. The products will be opposed-cylinder engines, and will use roughly half the parts of the average automobile engine. Gates' contribution has given the firm the funds it needs to test and produce a prototype in the coming months. Through the Bill & Melinda Gates Foundation, Microsoft's founder has pledged support to a variety of philanthropic initiatives around the world. In 2010, one of the organization's focuses has been Africa. A partnership with Coca-Cola ( KO) is designed to benefit fruit farmers in Uganda. Gates has visited Nigeria twice over the past two years, pledging at least $150 million to fight polio there. Beyond simply donating medicine and food, Gates has taken an interest in helping African farmers and other workers in improving the viability and profitability of their small businesses. Gates seems committed to helping Africa help itself, and he has the money to make quite an impact (especially in the wake of a recent pledge by a number of American billionaires to give away half their wealth to charity). Finally, Gates, along with fellow billionaire Warren Buffett, has been investing consistently in Republic Services ( RSG) over the past three years. Gates alone holds nearly $2 billion (roughly 66 million shares) in this waste management firm.