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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.

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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.

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 Forbes estimates Allen’s net worth at $14.2 billion. Wikimedia Commons/ Michael Sprague 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 toreplicate the brain and build machines with human intelligence.

Vinod Khosla: Fake meat and cheese Owen Thomas, Business Insider Hamburger is a $100 billion market just waiting for technology to disrupt it. So says top venture capitalist Vinod Khosla, who is funding a stealth startup that turns protein, fat, and fiber from plant sources into a juicy, beefy-tasting patty. “There are no cows involved, but you can’t taste the difference,” he promised. He’s also backing a startup working on fake cheese; one working on less-salty salt; and Unreal, a candy maker which promises healthier sweets.

Peter Thiel: Living forever Fortune Live Media / flickr Venture capitalist Peter Thiel, a billionaire thanks largely to his early backing of Facebook, is financing medical research that could help people live to be 150 years or more. “There are all these people who say that death is natural, it’s just part of life, and I think that nothing can be further from the truth,” Thiel has said. The Thiel Foundation contributes to a number of anti-aging research projects, including Ray Kurzweil’s Singularity Institute, which works on artificial intelligence, and Aubrey de Grey’s Strategies for Engineered Negligible Senescence, which hopes to reverse aging altogether.

Sergey Brin: Curing Parkinson’s In 2008, Google cofounder Sergey Brin revealed a surprisingly personal story about his mom having Parkinson’s disease. Through 23andMe, a genetic-testing startup cofounded by his wife, Anne Wojcicki, Brin discovered he has a good chance of getting Parkinson’s, too. But the startup also made a medical discovery that could save his life. 23andMe is now backed by Google’s venture-capital arm, Google Ventures, and others. Its mission, while broadly looking at gene-linked diseases, includes finding a way to save Brin and others from ths terrible illness.

Jeff Bezos: Millennium clock and a trip to the moon Amazon CEO Jeff Bezos is spending $42 million to build a giant clock that will chime every year, decade, century, and millennium—for the next 10,000 years. He’s got other interesting pet projects too, like Blue Origin, a space-travel company that wants to take astronauts to the International Space Station. In 2011, he hired undersea experts to find the engines that sent the Apollo 11 rocket into space. In March 2012, they succeeded in locating them.

Michael Dell: Historical photo collection Dell.com Michael Dell run a charitable foundation with his wife, Susan, dedicated to helping kids in urban poverty. That’s a wonderful mission. Dell also puts his money into more offbeat causes. In 2010, his private investment company, MSD Capital, bought the Magnum Collection. It includes more than 180,000 of some of the nation’s most important and memorable photographs taken since 1947. He houses the collection at the Ransom Center museum in Austin, Texas.

Marc Benioff: Fetal surgery Marc Benioff Wikipedia Marc Benioff is famous for his 1-1-1 philanthropic model in which companies dedicate a percentage of profits, equity, and employee time to nonprofit work. As Benioff points out, this differs from the another popular method, like The Giving Pledge, in which billionaires collect money throughout their lives and then give it away at the end. Benioff has a lot of pet projects, especially around his beloved city of San Francisco. But he’s best known for his $100 million donation for the 180-bed UCSF Benioff Children’s Hospital. He was drawn to UCSF because of its “pioneering work with fetal surgery,” he said in an interview with Forbes.

Dustin Moskovitz: Malaria and parasite worms Wikipedia Dustin Moskovitz, a billionaire Facebook cofounder who went on to launch an enterprise startup, Asana, wants to solve poverty-related disease. His foundation, Good Ventures, gives money to causes recommended by GiveWell, a nonprofit run by Moskovitz’s girlfriend, former Wall Street Journal reporter Cari Tuna. Good Ventures donated $500,000 to fight malaria and another $250,000 to fight schistosomiasis, a disease caused by parasitic worms.

David Sacks: Socially relevant films Yammer CEO David O. Sacks Wikimedia, CC. Yammer founder David Sacks, a member of the PayPal Mafia, is also a Hollywood hitmaker. Through his production company Room 9, Sacks produced the film “Thank You For Smoking,” which grossed some $39 million worldwide. That wasn’t Room 9’s only film. It also produced “Dumbstruck,” a movie about professional ventriloquists. Sacks is also interested in startups that are taking on older industries that haven’t really figured out the Internet yet. He invested in Uber, which successfully disrupted the taxi industry. But another investment, Cherry, shut down its mobile car-wash service last month.

Hasso Plattner: Fighting AIDS, improving design SAP founder, chairman, Hasso Plattner Business Insider Billionaire SAP cofounder Hasso Plattner invested $35 million into the Hasso Plattner Institute of Design at Stanford University, focused on product design. He also gave about $265 million to the Hasso Plattner Institute, which focuses specifically on enterprise-software design. He’s also donated millions to fighting AIDS. He hosts concerts to raise AIDS awareness at his luxury golf estate in South Africa, a country hard-hit by the disease. He’s also known as another sailboat racing enthusiast whose pro racing team has faced-off with arch business enemy Larry Ellison. He once even mooned Ellison on the high seas—or at least one of Ellison’s boats, at any rate.

John Chambers: Ending cancer Cisco CEO John Chambers CNBC Cisco CEO John Chambers wants to kill cancer dead. A year ago he donated $750,000 for cancer research at West Virginia University. Both of his parents were doctors who graduated from WVU. Chambers has been known to fight cancer in more personal ways, too, particularly when it strikes Cisco employees. Chambers once personally arranged a second opinion for a Cisco employee who was diagnosed with a terminal, inoperable form of cancer, a former Cisco exec told us. That employee got the surgery and survived.