Newmark started e-mailing friends a list of San Francisco events in 1995. It became the basis of Craigslist, the Internet’s most popular classified ad site. While most of the listings are free, it does charge for job postings in 18 cities and brokered apartments in New York. Analysts figure the site, which is available in 70 countries and gets more than 20 billion page views per month, could be worth $3 billion if it charged for postings in all cities, ran ads and cleaned up listings.
Net Worth: $400 million