mark shuttleworth got his hundreds of millions (billions?) from verisign. verisign sells ssl certificates. an ssl certificate is esentially a random number tied to a name and address in their database.
mark made his money selling random numbers to people. correction - leasing random numbers. it costs $50/year and up to lease a random number. not anyone can generate random numbers like these though, these numbers are tied to another random number, the certificate authority(CA). verisign is a CA and back in the days before internet explorer, they got their CA into the source code of IE. That means IE accepts unequivocally the certificates from verisign where as others make IE pop up a "warning! are you sure?" message. there are other CAs in IE besides verisign, but verisign is the largest. How did microsoft select this list of CAs? I'm sure verisign knew what a goldmine it would be to get its CA certificate on that list, with no downside if it didn't become popular. Its very hard to compete with verisign at this point because all those millions of older copies of windows have the original list of CAs.
luckily, mark did something very very good with his money. he created collabnet, which funds ubuntu. ubuntu the single best and subsequently most popular linux desktop effort currently in existence.