Slate has a list of the 60 largest American donations to charity in 2005. There are several interesting things about this list.
While there are plenty of familiar names - Bill Gates and Paul Allen of Microsoft, George Soros, Michael Bloomberg, Ted Turner of CNN, Oprah Winfrey, the odd Rockefeller and Ford - quite a few of those listed are much less well known. This suggests that their money is probably inherited ("family wealth" as the list delicately puts it) rather than earned through any newsworthy current ventures of their own.
Also noticeable is the absence of many other enormously bloated individuals who could easily spare a few million bucks: no Warren Buffett, no Larry Ellison, no Steve Jobs, none of Sam Walton's heirs (of course they could appear further down the top 100 list, and some have made big donations in the past). Essay question: compare and contrast with the Forbes rich list.
Another observation is that many of the donations are to already massively wealthy institutions: MIT, UCLA, Princeton, Stanford, Wellesley College, (probably most of these are the alma mater of the donor), the Smithsonian, the Museum of Modern Art, the Ronald Reagan Presidential Library - rather than the less glamorous but infinitely more vital needs like clean water supplies and childhood vaccination in the Third World, medical aid for war victims (no shortage of those thanks to America's foreign policy), rainforest preservation and [name your own cause here].
Final note: much as I admire the work of the Gates Foundation, which does address some of these causes, I can't help feeling that the best service Gates and Allen could do for the world would be to provide more reliable software at a more affordable price!