Dozens of the country's wealthiest are promising to share their riches as part of a charity effort by billionaires Warren Buffett and Bill Gates.

Gates and Buffett launched "The Giving Pledge" campaign six weeks ago to encourage the rich to give more, as the pair have done in the past.

So far, forty billionaires from across the U.S. have pledged to give away half their fortunes to charity. Collectively, nearly $250 billion in pledges have been placed -- about $1,000 per person in the U.S.

New York City Mayor Michael Bloomberg participated in the effort and said he believes it's a worthy cause.

"You can't spend it if you have over a certain amount," he said. "And if you really care about your family, I've always thought that the best thing to do is to make the world better."

The pledges are a moral commitment, not a legal contract.

Buffet and Gates hope The Giving Pledge will motivate other Americans to give to charity as well. The effort was inspired by everyday people who give time and money to charity, even when they can barely afford to.

The Giving Pledge Web site includes a list of donors so far.

*Originally published August 5, 2010.