If you'd founded a tech company worth billions of dollars, what would you do with your cash?
That's a high-class problem faced by the moguls on this list.
Naturally, many of them buy expensive things, like cars, houses, planes—even islands.
But even if you're Larry Ellison, with a seemingly endless appetite for that stuff, there comes a time when you want to do more.
Maybe it's solving the world's problems, or just indulging in a geeky fantasy.
Larry Ellison: Anti-aging
Oracle cofounder and CEO Larry Ellison is known for pet projects like running the America's Cup sailboat race series, buying homes on Malibu's "Billionaire's Beach," and turning the Hawaiian island of Lanai into a sustainable living lab.
But he says his biggest philanthropic endeavor is medicine.
"I have a medical foundation called, very creatively, the Ellison Medical Foundation," Ellison said in an interview at the D: All Things Digital conference last year. "We are focused on diseases related to aging—I mean, for obvious reasons." (Ellison is now 68.)
Ellison spent about $1 billion on this foundation.
Bill Gates: High-tech toilets
Through the Bill and Melinda Gates Foundation, the Microsoft cofounder is working on a lot of social issues in health, education, and agriculture.
But one problem that really caught Gates' attention is ... poop. The foundation sponsored a "Reinvent the Toilet" fair. Gates himself judged the entries and announced the winners.
He was looking for methods "for capturing and processing human waste and transforming it into useful resources," Gates said in a blog post announcing the winners.
Paul Allen: Brain research, sports teams, and collectibles
Paul Allen, Microsoft's other billionaire cofounder, also has a long list of pet projects that includes owning multiple pro sports teams, building a rock-and-roll museum, collecting vintage WWII planes, and more.
He's also invested a half billion dollars into the Allen Institute for Brain Science. Its mission is to figure out exactly how the brain works and how to solve diseases like Alzheimer's, which his mother suffered from.
Another goal of the institute is to replicate the brain and build machines with human intelligence.
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