40 under 40

9. John Arnold

With an estimated net worth of more than $3 billion, the master energy trader is the second youngest American billionaire (see No. 2). His Houston-based $5 billion hedge fund is one of the decade's most successful, returning more than 200% in some years (though it is said to be having a lackluster year). The former Enron trader also invests in hard assets like underground storage caverns for natural gas, which will make him an even larger figure in this burgeoning market.

Big giver: Arnold and his wife recently took the Warren Buffett-Bill Gates pledge to give half their wealth to charity.

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Last updated October 14 2010: 1:25 PM ET