Each year, Charles and David Koch reinvest 90% of the profit in the business, with enough left over to pour hundreds of millions of dollars into their favorite charities and causes, a mix of libertarian think tanks and New York City arts institutions. Most recently, they angered California Gov. Arnold Schwarzenegger by giving $1 million to help efforts to overturn the state's climate change regulations.

No. 5 (tie): David Koch, $21.5 billion

Sources of wealth: manufacturing, energy

Residence: New York

Age: 70

2009 rank: No. 9 (tie)

More gregarious than his brother Charles, David Koch may have made his shrewdest decision in 1983, when he kept his stock in Koch Industries instead of selling out like his brothers William and Frederick, who got about $790 million for their stakes.

Since then, the company has expanded rapidly and now is worth more than $50 billion; it has interests in pipelines, refineries, Lycra and Dixie Cups.

David, who was the Libertarian Party's candidate for vice president in 1980, now restricts his political activities mostly to supporting conservative think tanks and activist organizations. From his home base in New York City, he runs Koch's chemical technology group. He and his wife, Julia, are also active on the charity circuit and have given or pledged $600 million, mostly to cancer research and the arts, since 2000. He sits on 26 nonprofit boards.

No. 7: Jim Walton, $20.1 billion

Source of wealth: Wal-Mart Stores

Residence: Bentonville, Ark.

Age: 62

2009 rank: No. 5

Jim Walton is the chairman and chief executive of family's Arvest Bank; he also chairs Community Publishers, a local-newspaper company. Sam Walton's youngest son has served on Wal-Mart's board of directors since his brother John's death in 2005.

Although Wal-Mart's shares have been nearly flat over the past year, the three surviving children of founder Sam Walton collected $1.2 billion in dividends.

No. 8: Alice Walton, $20 billion

Source of wealth: Wal-Mart Stores

Residence: Fort Worth, Texas

Age: 61

2009 rank: No. 6

Alice Walton is building the Crystal Bridges Museum of American Art in Bentonville, Ark. The daughter of Wal-Mart founder Sam Walton, she was an equity analyst after college and later formed investment bank Llama and led the creation of Northwest Arkansas Regional Airport. The horse lover also runs Rocking W Ranch in Texas.

No. 9: S. Robson Walton, $19.7 billion

Source of wealth: Wal-Mart Stores

Residence: Bentonville, Ark.

Age: 66

2009 rank: No. 7

S. Robson Walton has been the chairman of Wal-Mart since his father's death in 1992. The Columbia Law School grad joined the company in 1969. The positions he's held: senior vice president, secretary and general counsel, and vice chairman.

No. 10: Michael Bloomberg, $18 billion

Source of wealth: Bloomberg LP

Residence: New York

Age: 68

2009 rank: No. 8

America's richest politician, Michael Bloomberg only recently saw his campaign spending record of $109 million surpassed, by California gubernatorial candidate Meg Whitman. He is serving his third term, set to expire in 2013, after successfully campaigning for extended term limits in 2008.

The Boston-born son of a Russian immigrant accountant is registered as an independent, but he is a social liberal. In a recent address, he said: "I'm pro-choice. I'm pro-gay rights. I'm pro-immigration; I'm pro-gun control. I believe in Darwin."

Outside politics, the mayor owns an 88% stake in Bloomberg LP, a data analytics company he founded in 1982 after being fired from Salomon Brothers. ("Living well is the best revenge," he recently said of his multibillion-dollar turnaround.)

While other financial-media companies have been cutting jobs and selling titles, Bloomberg is on a growth spurt. The company, which operates about 290,000 data terminals, has increased its employee head count 25% over the past three years; it picked up BusinessWeek for less than $5 million. Overall revenue is up an estimated 10% this year to $6.9 billion, thanks in part to the company's expansion into the legal-data field.

Michael Bloomberg has given more than $1.4 billion to charities over the years; his causes include anti-smoking, public health and the arts.

In a letter accompanying his promise to donate the bulk of his net worth through the Giving Pledge, he explained why he chooses to give money now rather than bequeath his fortune: "Why wait? Why deny financial aid to this generation? Why deny a possible cure for a disease to this generation?"