From the Forbes 400 list, the following are the billionaires who made their money on businesses whose products are primarily free to consumers:
(Note: I didn't include diversified media tycoons, such as Rupert Murdoch and Barry Diller, even though much of their business is free-to-air broadcast and web media. That's because it's too hard to separate the free bits from their pay-media businesses, or to say which is bigger.)
#13 Sergey Brin $15.9 billion, Google
#14 Larry Page $15.8 billion, Google
#54 Pierre Omidyar $6.3 billion, eBay
#59 Eric Schmidt, $5.9 billion, Google
#155 Oprah Winfrey, $2.7 billion, free-to-air TV
#161 Mark Cuban, $2.6 billion, Broadcast.com
#246 Omid Kordestani, $1.9 billion, Google
#246 Joseph Mansueto, $1.9 billion, Morningstar (freemium investing services)
#281 David Filo, $1.7 billion, Yahoo
#281 Jerry Yang, $1.7 billion, Yahoo
#281 Kavitark Ram Shriram, $1.7 billion, Google
#321 Todd Wagner, $1.5 billion, Broadcast.com
#321 Mark Zuckerberg, $1.5 billion, Facebook
#377 Peter Thiel, $1.3 billion, Facebook, Paypal
Add it up and that's more than $62 billion of net worth built on free. And that's just within the top 400 Americans.