Michael Bloomberg: $40 billion

Created the eponymous financial information provider in 1981 after getting laid off from an investment bank. Bloomberg made it a lucrative business in particular by selling data terminals to financial services firms. The multi-screen terminals became essential tools in the industry, incorporating real-time market information with a news service. Bloomberg, who reportedly retains an 88 percent stake in the privately held company, turned to politics in 2001, becoming mayor of New York City for three terms.