Sara Blakely and eight other billionaires joined Warren Buffett and Bill and Melinda Gates' Giving Pledge, promising to devote at least 50% of their wealth to philanthropy.

By Carol J. Loomis, senior editor-at-large

Sara Blakely, founder of Spanx

FORTUNE -- Nine new signers of The Giving Pledge -- the philanthropic effort initiated three years ago by Warren Buffett and Bill and Melinda Gates -- were announced today, and one of the billionaires signing is the 42-year-old founder of lingerie company Spanx, Sara Blakely. She starts off her letter -- to be posted on the Giving Pledge website -- in a way no other signer will be able to emulate.

"Since I was a little girl," she says, "I have always known I would help women. In my wildest dreams I never thought I would have started with their butts."

She refers to the start of Spanx, which got its inspiration when she needed a smooth undergarment to wear beneath a pair of dress pants and got the idea of cutting off the feet of a pair of control-top pantyhose and donning those. The only problem: The bottoms rolled up all night.

MORE: Inside the boardroom: The party is over!

So next day she began searching for a way to have the garment be perfect high and low -- and that was the start of Spanx, founded in 2000. Today, Blakely's company, still owned 100% by her, is estimated to have annual sales of $250 million and very big profit margins.

Deciding to join The Giving Pledge, she says she sat her 3 1/2-year-old son Lazer down and told him she was going to give away at least half of her money -- and that she hoped one day he would understand and think she had done the right thing. "Okay, Mommy," he said, "can we do a puzzle now?"

Here are the eight other signers -- some who have signed as married couples -- who have promised, in the mode of The Giving Pledge, to contribute at least 50% of their net worth to philanthropy.

The group includes two men who are not Americans, Lord Ashcroft, of the U.K., and Samuel Yin, of Taiwan.

Michael Ashcroft, as he was known in earlier decades, was a serial buyer and seller of companies in many geographies. Running a company called Hawley Goodall, he bought Crime Control of Indianapolis in 1987 and then added ADT Security Services, building a very large security company.

His American experience, says Lord Ashcroft in his pledge letter, led him to discover "one of the most appealing traits of American life … the tendency of many wealthy individuals to see it as part of their civic duty to support charities." His first major charity was forming Crimestoppers, which has led to 120,000 arrests.

Samuel Yin built the Taiwanese company Ruentex, which is an integrated network engaging in textile manufacturing, retailing, marketing, and trading. The company has a market value close to $2 billion. Like many pledgers, he plans to give much more than 50% of his net worth to charity -- his goal being 95%.

Joe Craft, another of the pledgers, is one of many members of The Giving Pledge who grew up in impoverished areas -- in his case, Hazard, Kentucky. Then coal brought jobs to Appalachia, and he went on to form coal's first publicly traded master limited partnership, Alliance Resource Partners (ARLP). He says, "Basic tenets of American enterprise -- hard work, financial discipline, and teaming up with great people -- have allowed me to be in the position to take this pledge."

MORE: Apple: What it's like to drown in cash

Monica and David Gelbaum say in their unusual pledge letter that they are acting after the fact: "We have given in excess of $1 billion and are not in a position to give more." Their fortune came from Entech Solar (ENSL). They say that about 70% of the upside in Entech, including all of their interest in the company, has been pledged to philanthropies for the benefit of veterans and military families.

Craig and Susan McCaw: He is an entrepreneur who built McCaw Cellular into the largest U.S. cellular company, and she is president of COM Investments and has served as U.S. Ambassador to Austria. Their foundation has supported educational, environmental, and international development projects.

Prominent in real estate, Stephen M. Ross founded Related Companies and today is the owner of the Miami Dolphins. In his pledge letter, he says "I turned a 10-page business plan" -- created after he'd been fired by a New York financial firm -- "into over a $15 billion business." Formerly private in his philanthropy, he says he joined The Giving Pledge hoping to inspire others to give.

Paul Singer founded and heads Elliott Management Corp., a New York-based trading firm that manages $21 billion. In his philanthropy, he has supported research and scholars in the areas of free-market economics, rule of law, health care delivery innovation, U.S. national security, and the future of Israel.

Mark and Mary Stevens: He is a former managing partner of Silicon Valley's famous venture capital firm Sequoia Capital. He and his wife describe themselves as "in the early innings" of their philanthropy. They have recently turned to, among other causes, health care research and health care delivery. Giving their excess wealth to their three kids, they say, would have inhibited their children's "dreams and motivations."

MORE: Who could succeed Jamie Dimon?

Ted Taube is president of Koret Foundation and formerly ran women's sportswear company Koret of Califorina. He was born in Poland and immigrated to the U.S. with his parents on the eve of the Nazi invasion of Poland. "We were survivors in a land of opportunity," he says in his pledge letter. But he is not too sanguine about the direction of the U.S. today and is therefore giving to education reform, public policy programs, and constitutional education workshops.

The people listed in this article bring the total number of signers of The Giving Pledge to 114.