Vanguard Vanguard founder John Bogle.
Warren Buffett just published his annual letter to Berkshire Hathaway investors, and buried in the 29-pages is a shoutout to one Jack Bogle.
"If a statue is ever erected to honor the person who has done the most for American investors, the hands- down choice should be Jack Bogle," Buffett writes, adding:
"For decades, Jack has urged investors to invest in ultra-low-cost index funds. In his crusade, he amassed only a tiny percentage of the wealth that has typically flowed to managers who have promised their investors large rewards while delivering them nothing - or, as in our bet, less than nothing - of added value.
"In his early years, Jack was frequently mocked by the investment-management industry. Today, however, he has the satisfaction of knowing that he helped millions of investors realize far better returns on their savings than they otherwise would have earned. He is a hero to them and to me."
In 1974, Bogle founded Vanguard Group, which transformed investing forever with the index fund.
The index fund, which passively tracks a market index, allows ordinary people to invest in the stock market at much lower fees than in traditional, actively managed funds.
Arthur Levitt, former chairman of the US Securities and Exchange Commission, has called Bogle "a visionary in global finance." Paul Samuelson, the Nobel prize winning economist, said he ranked the invention of the index fund up there with the "invention of the wheel, the alphabet, Gutenberg printing, and wine and cheese."
The amount of money put into index funds has exploded over the years. Vanguard is also now the leader by far - something Bogle didn't expect. The firm manages $3.9 trillion.
"It was never my design to build a colossus," Bogle, 87, told Business Insider in an extensive interview earlier this year.
The first is Vanguard's ownership structure, which differs from most in that Vanguard doesn't have outside shareholders. The company is owned by its funds, which are in turned by their shareholders.
That means Vanguard is able to return profits to fund shareholders through lower costs. It's part of the reason why some of predicting that exchange-traded fund fees may drop to zero in 2017.
The other revolutionary idea is the index fund.
"It took 20 years before it started to catch on in the mid-1990s, and now it's dominating everything we say in this financial field," Bogle said. "It's changing it forever.
Vanguard has shaken up the industry, and accounted for the majority of inflows into mutual funds since 2014, according to Morningstar data.
Vanguard now dwarfs rivals in raking in assets from investors.
Vanguard accounted for the majority of inflows into mutual funds since 2014, according to Morningstar data. 
Despite his stature within the money management industry, Vanguard's structure means Bogle has made a pittance compared to active managers and other CEOs.
In a 2012 interview with the New York Times, Bogle estimated his wealth in the "low double-digit millions." By contrast, Abigail Johnson, who heads Fidelity Investments, is worth $14.1 billion, according to Forbes.
Read Business Insider's full interview with Bogle here.