In response to a call from Warren Buffett and Bill Gates for billionaires to give away half of their fortunes to charity, former Microsoft exec and entrepreneur Paul Allen announced last week that he planned to give most of his fortune, estimated to be $13.5 billion, to charity.

Perhaps that rallying call to philanthropy is being reflected in larger gifts this year by multimillionaires, according to The Chronicle of Philanthropy.

“The donations that have been made show that many wealthy Americans are more willing to part with large sums than they were a year ago,” the Chronicle reported. “At least 20 people have made gifts of $20 million to $35 million in the past six months; in the first part of 2009, only 13 such gifts had been made.” And three gifts of $100 million or more were made this year compared with only two in the first six months of last year.

“I think it’s a sign that people are more confident in the economy. Those who are willing to give are more confident but it’s a mixed picture,” Stacy Palmer, editor of the Chronicle, said. “And some of those people giving those bigger gifts are sending a signal to other wealthy people.”

While the wealthiest are giving bigger gifts, fewer people over all are making donations of more than $1 million, according to the Chronicle. Only 181 gifts of $1 million or more were given out in the first six months of 2010 compared with 250 for the same time period last year.

Philanthropic organizations report that uncertainty over the estate tax is keeping many donors from even having a preliminary discussion with them, Palmer said. Furthermore, bequests to charity are taking longer to distribute when they involve real estate because of the weak market.

At the same time family foundations are paying out more than the 5% of assets required by law, according to the Chronicle. Small and midsize foundations, which are required by the IRS to give away only 5% of their assets, generally gave away more than that this year, according to a study by Foundation Source, a company that provides services for family foundations and other philanthropies.

Family foundations were more generous last year, when the number and size of grants less than $1 million increased by 15%. Family foundations are required by the IRS to give away a minimum of 5% of assets annually, but last year they gave more.

“The numbers are kind of tricky,” Palmer said, “but the fact is that they’re giving more, and in this economy, groups need it more. The asset bases are actually smaller as well,” so it may be that in order just to keep up with donations from the past, foundations have to give out more than 5%.

The 5% rule is based on the amount deemed necessary to keep the foundation going forever, but the increased giving may be “ the family thinking that it’s more important to give today than to ensure that the foundation will live forever,” Palmer said.