#2 Joe Green took his dad's advice and said no to Facebook

Mark Zuckerberg's college roommate, Joe Green, turned down an offer to help Zuckerberg start Facebook.

When the two were at Harvard, they created a Hot-or-Not style website called Facemash, which got the pair in trouble with the university. When Zuckerberg asked Joe Green to help him with Facebook, Green's dad discouraged his son from doing another project with him.

Had Green joined the company in those early days, he would have gotten about a 5 percent stake, he thinks, which today would be worth about $3 billion.

#1 Nolan Bushnell could have owned one-third of Apple

Nolan Bushnell, co-founded Atari, a video game and home computer company, turned down the opportunity to invest $50,000 in seed money in Apple. At Atari, Bushnell was one of Steve Jobs' first bosses.

Had Bushnell said yes, he would have owned a third of Apple, a company that is today valued at more than $400 billion.

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