EDIT: As the post below me says, this is voluntary. Now that I think about it, it's that one they ask you about when you file your income tax, the $5 one or whatever.
"Releasing the charitable impulses of the American people by freeing them from the excessive tax burden so they can devote more of their resources to charity, is a moral and constitutional means of helping the needy. By contrast, the federal welfare state is neither moral nor constitutional. Nowhere in the Constitution is the federal government given the power to levy excessive taxes on one group of citizens for the benefit of another group of citizens. Many of the founders would have been horrified to see modern politicians define compassion as giving away other people's money stolen through confiscatory taxation. In the words of the famous essay by former Congressman Davy Crockett, this money is 'Not Yours to Give.'"