Bill gates seem to be far the most generous of them :
-Bill and Melinda Gates Foundation
-The Giving Pledge
The official website states that it "is an effort to invite the wealthiest individuals and families in the United States to commit to giving the majority of their wealth to philanthropy." As of April 28, 69 billionaires in the US have joined this campaign and pledged to give 50% or more of their wealth to charity. A minimum total of at least $125 billion has been promised from the first 40 donors based on their aggregate wealth as of August 2010.