In 1993, an unknown engineering student at the University of Illinois helped create a free software tool that made it much easier to access information on the Internet. He moved to Silicon Valley, met Jim Clark, and founded Netscape. Clark and Andreessen released the Netscape Navigator in 1994, and the pair took the company public a year later. Netscape's stock opened at $28 a share and shot up to $75 on its first day, igniting the tech boom with Andreessen as its 24-year-old poster boy. He appeared on the cover of Time magazine in 1996 in his bare feet and sitting on a throne.

Netscape would eventually succumb in the infamous Browser Wars to Microsoft -- though not before Andreessen sold the business to AOL for a whopping $4.2 billion for the company. Netscape also managed to spin off its software, which became the foundation for Mozilla, the not-for-profit behind the Firefox browser. Andreessen stepped down from his role at AOL in 1999 telling the New York Times that he wanted to work on start-ups. "They are about breaking the rules and trying new things that might fail or might change the world," he said. A month later, he founded Loudcloud, promising to make software for web servers. He took the company public in 2001, and then sold it to HP for $1.6 billion in 2007. His latest company, Ning, allows people to create their own social networks.

In addition to founding three companies, Andreessen has also proven himself capable as an angel investor -- he made seed investments in recent standouts like Digg, Twitter, Facebook -- and as a writer. His blog, Pmarca, includes Andreessen's thoughts on the economy, company building, and fundraising. Our favorite post offered advice on what to do when investors say no: "One 'no' doesn't mean anything--the VC could just be having a bad day," he wrote. "Or she had a bad experience with another company in your category, or she had a bad experience with another company with a similar name, or she had a bad experience with another founder who kind of looks like you, or her Mercedes SLR McLaren's engine could have blown up on the freeway that morning--it could be anything. Go meet with more VCs."