Fortune Magazine (search) didn't have to shop around to find the most powerful woman in the United States — she's Meg Whitman, the 49-year-old chairman and CEO of eBay (EBAY).

For the second year in a row, she tops the list of the magazine's 50 Most Powerful women.

And she practices what she preaches.

She bought seven new businesses this year for a total of $1.3 billion.

In second place is Anne Mulcahy (search), 52, the chairman and CEO of Xerox (XRX), who helped boost the company's net income by 138 percent.

Making the biggest leap is Brenda Barnes (search), the 51-year-old president and CEO of Sara Lee (SLE), who rocketed to No. 3 from 39th place last year, thanks to her promotion to head the troubled $19 billion company.

Oprah Winfrey, 51, rose two positions, to No. 4, thanks to her "awesome cultural influence" as chairman of Harpo Productions, which includes her TV show with 9 million viewers a day and her latest magazine success, "O at Home."

And Martha Stewart is back in the power seat, despite being a convicted felon who served time in the slammer.

After her lies to the Securities and Exchange Commission got her kicked off the list for a year, the 64-year-old domestic diva has climbed to the No. 21 spot.

As founder of Martha Stewart Living Omnimedia, "her position doesn't begin to hint at her command," Fortune says.

"Her cultural clout remains remarkable."

In ninth place is Karen Katen, 56, who heads the Human Health division of Pfizer, which produces 88 percent of revenues for the world's biggest drugmaker.

She also has been with Pfizer longer than the tenure of any other name on the list: 31 years.

Eight women have made the list every year since Fortune started it in 1998: Oprah; Katen; Hearst president Cathleen Black, 61, (No. 34); Avon CEO Andrea Jung, 47, (No. 5); Ogilvy & Mather CEO Shelley Lazarus, 58, (No. 26); MTV CEO Judy McGrath, 52, (No. 10); Time CEO Anne Moore, 55, (No. 13); and Lucent CEO Pat Russo, 53 (No. 14).