The Idealists

In 2007, while working as a Wall Street trader, Brad Katsuyama decided the stock markets were rigged. He thought people were gaming the system using what's now known as high-frequency trading.

So Katsuyama launched a first-of-its-kind business called the Investors Exchange, or IEX. It aims to level the playing field with a 350-microsecond latency on trades, among other things.

IEX opened for trading in October 2013. (It was the subject of Michael Lewis' controversial book Flash Boys: A Wall Street Revolt.) It's gone from trading around 30 million shares a day in March 2014 to over 140 million shares some days.

Katsuyama has raised $101 million from investors like Spark Capital and Bain Capital Ventures.

IEX now has 58 employees, and it's on schedule to become a stock exchange this year, according to Katsuyama, 37.

"A lot of my career has been spent parachuting into a problem, and working out of it through building solutions," he said. -- S.O.