David Koch
New York, N.Y.
Net Worth: $17 billion
In 2004, Koch was said to have bid $17 million for an 18-room duplex in 740 Park Ave., one of Manhattan's most exclusive buildings. He's since spent millions more on extensive renovations. He's in good company: The 17-story building is also home to George David, head of United Technologies, who paid $25 million for his place in 2004, and financier Steven Schwarzman, who bought part of John D. Rockefeller's old apartment.
J. Christopher Flowers
New York
Net Worth: $2 billion
Investment banker J. Christopher Flowers scooped up the Harkness Mansion on East 75th Street for $53 million in 2006, making it the most expensive single residential sale in Manhattan history. The 22,000-square-foot, neo-French Renaissance townhouse, built in 1896, was formerly owned by producers Jaqui Safra and Jean Doumanian, who bought the property in 1987 for $6.9 million.
Sumner Redstone
Beverly Hills, Calif.
Net Worth: $7.6 billion
The Viacom head reportedly spent about $15 million on this spread in the exclusive Beverly Park gated community--not a bad bicoastal complement to his apartment in the exclusive Pierre Hotel in New York City. Built in 1997, Redstone's home measures about 15,000 square feet and has three bedrooms, a screening room, an outdoor pool and tennis courts. Neighbors include Denzel Washington, Eddie Murphy and Paul Reiser.
Steven Spielberg
East Hampton, N.Y.
Net Worth: $3 billion
Quelle Farm, Steven Spielberg's summer retreat in the Hamptons, has good--or at least expensive--company. It sits on Georgica Pond, an area that is a who's who of powerful moguls. Other high-profile neighbors include financier Ron Perelman and designer Calvin Klein. Needless to say, these aren't Spielberg's only digs; he also has homes in Manhattan and Pacific Palisades, Calif. 
Michael Bloomberg
New York, N.Y.
Net Worth: $11.5 billion
Gracie Mansion, New York's mayoral residence, is one of New York's finest properties. But billionaire mayor Michael Bloomberg prefers his own luxurious digs. Built in 1889, this 7,500-square-foot townhouse (center, left) on East 79th Street is steps from Central Park and boasts five floors. In 1986 it set Bloomberg back $3.5 million. Hizzoner also owns properties in Bermuda; London; Vail, Colo.; and North Salem, N.Y.
David Geffen
Beverly Hills, Calif.
Net Worth: $6 billion
When Geffen splurged on this Beverly Hills estate, he set a Los Angeles real estate record. The music mogul paid $47.5 million for the 100-room mansion, which has its own three-hole golf course. The real kicker? That was 15 years ago. Geffen later explained that the home, owned by late studio head Jack Warner, included a trove of art and antiques. Geffen's real estate holdings also include a Malibu beach home.
Craig McCaw
Hunt's Point, Wash.
Net Worth: $2.8 billion
Billionaires who grow up in the Puget Sound area tend to stay local. McCaw, who was raised in a 20,000-square-foot home in Seattle, is one of them. Several years ago, he reportedly paid $20 million for this English manor-style estate built by Kenneth Gorelick (aka musician Kenny G). Hunt's Point pokes into Lake Washington, whose waters also wash onto the private beaches of Bill Gates and Paul Allen.
Tamir Sapir
New York, N.Y.
Net Worth: $1.5 billion
The seven-story Duke-Semans mansion faces the Metropolitan Museum of Art on Fifth Avenue and boasts a mansard roof and petaled glass marquee. Sapir, a cab driver turned real estate and oil baron, Sapir bought the historic property from the relatives of tobacco heiress Doris Duke for $40 million after visiting it only once, and experts say he could spend as much as $10 million renovating it.
Donald Trump
Palm Beach, Fla.
Net Worth: $3 billion
The Mar-A-Lago estate, which Trump bought in 1985, may not have 475 feet of ocean frontage like Trump's $125 million Maison de l'Amiti�. Still, it isn't too shabby. The 62,000-square-foot main house boasts 58 bedrooms and 33 bathrooms. South Florida weather is usually pretty warm, but in case it gets cold, the property has 12 fireplaces.
Michael Dell
Austin, Texas
Net Worth: $17.2 billion
Dell's 33,000-square-foot mega-mansion is perched on a hilltop in the college town where he founded his eponymous computer company. The property has views of the surrounding hills and of the Austin skyline, as well as Dell's dorm at the University of Texas. Though he reportedly spent tens of millions of dollars building the estate, in the late 1990s he wrangled with local assessors over its value, which was finally pegged at $12 million.
Paul Allen
Mercer Island, Wash.
Net Worth: $16.8 billion
Even though Allen's wealth exceeds the gross domestic product of most small nations, he prefers to make his home just outside of Seattle, where he grew up. The single mogul's mother also lives on the compound, which includes a 10,000-square-foot mansion, 400 feet of Lake Washington waterfront and a National Basketball Association regulation-sized gym (he owns the Portland Trailblazers, after all).
Oprah Winfrey
Montecito, Calif.
Net Worth: $2.5 billion
Winfrey is just one of the reasons Montecito is sometimes called "Moneycito." (Neighbor Ty Warner, who made a fortune from Beanie Babies, is another.) The self-made media mogul paid $50 million for the 23,000-square-foot Georgian-style home in 2001. Besides this 43-acre property, Winfrey's real estate portfolio includes a $6.2 million Chicago apartment and an $8 million Maui property in Hawaii.
Larry Ellison
Malibu, Calif.
Net Worth: $26 billion
The Oracle chief must like California's weather; his properties dot the coastline from San Francisco to Malibu. Ellison bought this So-Cal pad for $20 million in 2005 from music producer David Foster. The tennis court and pool are impressive on their own, but what makes them particularly noteworthy is that they are connected to the main house by a cable car that descends a fixed track to the lower level of the terraced property.
Ron Perelman
East Hampton, N.Y.
Net Worth: $10 billion
In a summer home enclave filled with remarkable residences, Revlon boss Ron Perelman has a particularly notable one. Perelman bought the 57-acre estate, called "The Creeks" and set on Georgica Pond, in 1990 for $12.5 million. While that seemed high at the time, we estimate that if he were to sell today, the land alone would command $70 million, the same price he got for his Palm Beach, Fla., house in 2004.
Warren Buffett
Omaha, Neb.
Net Worth: $52 billion
At the rate Buffett is giving away his money, it won't be long before his net worth matches the value of his home, a 6,000-square-foot gray stucco house he bought in 1958 for $31,500. Though he may live frugally, Buffett does maintain a security detail for the modest property, which proved invaluable in quashing a September break-in attempt. In 2004, Buffett sold one of his two Laguna Beach, Calif., properties; the one he kept is valued at about $4 million
Bill Gates
Medina, Wash.
Net Worth: $59 billion
Despite phasing out of his day job to focus on his philanthropic foundation, Gates remains true to his roots in Washington, where Microsoft is headquartered and where he lives with wife, Melinda. His 66,000-square-foot home is built into a hillside on the edge of Lake Washington, near Seattle. It includes a 60-foot swimming pool with an underwater music system, a domed library with two "secret" bookcase doors and a 1,000-square-foot dining room.
