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The causes vary widely, from a $500,000 gift to the nonprofit Plymouth Housing Group in Seattle, which seeks to provide low-income housing to homeless people, to $10,000 to replace a dance floor at the University of Idaho.

Stacy Palmer, editor of The Chronicle of Philanthropy, said Allen's penchant to pursue a vast variety of quirky projects - like the trip to space or the music museum - gives many nonprofits hope that he will fund projects that might otherwise go ignored.

"I think people at nonprofits are frustrated that there isn't more opportunity to do things like that," Palmer said.

Allen also has invested heavily in cultural icons that aren't traditional philanthropies. An avid sports fan, he owns basketball's Portland Trail Blazers, the Seattle Seahawks football team and Sporting News. A movie buff, he has his own production company, was a major backer of DreamWorks Animation and rehabilitated a favorite theater in his hometown.

Marita Sturken, a University of Southern California associate professor who studies American culture, compares Allen's range of investments to those of past tycoons and says, "He's projecting a new version of what our culture values (are), which is much more ... about a kind of embrace of things popular."

"It's about gaining cultural capital," Sturken says.

Like any good businessman, Allen seizes on opportunity. He contributed $100 million to develop the Allen Institute for Brain Science because he believes technology can be used to cull through the vast amount of data needed to better understand how the brain works. The inaugural Allen Brain Atlas project is working to create a revolutionary 3-D map of the brain.

Still, the reclusive bachelor billionaire, who turns 52 on Jan. 21, insists his projects are not just self-indulgent. His massive donation to brain research may also be motivated by a desire to leave a lasting legacy.

"I'm trying to do some things now with the Brain Institute and understanding the genetics of the brain. I'm not sure where that's going to lead but it could lead to some real breakthroughs in the understanding of the brain," he said. "Who knows?"

When he decides to fund other projects, Allen says he looks for the untested method, or the promising idea that no one seems particularly eager to back.

Richard Hutton tried for years in the mid-1980s to raise money for a documentary on evolution, but eventually gave up. Years later, he was sitting at his desk at Walt Disney Imagineering when he received a call from an old friend in public television who had unbelievable news: Allen had decided to single-handedly fund a series on evolution.

Hutton was floored. He spent three years on the series and now works for Vulcan Productions, Allen's independent film production company.

Allen has also invested billions in business pursuits, many centered on the idea of a "wired world," which would meld tasks like computing, accessing the Internet, making phone calls and watching television. But the grandiose vision hasn't spawned the next Microsoft.

His investment record is dotted with notable disappointments, including the controlling stake in troubled cable company Charter Communications Inc. Some analysts estimate that Allen may have poured as much as $8 billion into the company, which has seen its stock plunge amid an ongoing federal investigation into questionable accounting practices. Charter is continuing to deal with the fallout from the investigation.

"I think there's a perception that he has never been particularly successful with his investments," said analyst Matthew Harrigan, who follows Charter Communications for Janco Partners in Denver. Harrigan said many Charter stockholders are not impressed with the way Allen has used the power his controlling stake could wield.

Less common are Allen's successes, such as profitable investments in Ticketmaster, CNET and USA Networks, all of which he has sold. His investment company won't disclose how much he made on those deals.

Still, the billionaire has continued to put his money toward some technological pet projects. Last year, he unveiled the FlipStart, which weighs less than one pound and promises to be the world's smallest Windows XP computer. But the FlipStart has yet to hit store shelves, weighed down by technological hurdles and concerns about how best to market the gadget.

In his defense, Allen says when you invest on the scale that he did during the boom years you can expect your share of failures. But he also has more recently turned to more conservative investments, and he says he's learned from his earlier investment mistakes.

Those who have worked with Allen on both non-profit and for-profit ventures describe him as someone who may spend years developing a project - one who doesn't hesitate to write a check once he makes a decision.

A guitarist himself, Allen spent years collecting Hendrix memorabilia before finally building his museum tribute to his musical idol.

His intense interest in his projects doesn't end once the check has cleared.

Steve Patterson, Trail Blazers president, says to work for Allen is to learn to carry your BlackBerry wherever you go - and expect it to occasionally buzz with a message from the boss in the middle of the night.

Though he noted that Allen was not one to pinch pennies, Patterson was one of several people involved in Allen's projects who say they have recently been encouraged to be more responsible with their budgets. That resulted earlier this year in a decision to let the Allen-backed company that owned the Trail Blazers' arena, the Rose Garden, file for bankrupcty. The team was unaffected by the decision.

Allen says he isn't just investing to make money.

"I want to see good financial returns, but also to me there's the extra psychic return of having my creativity and technological vision bear fruit and change the world in a positive way," he says.

After underwriting a new approach to space travel or creating a museum that he imagines inspiring the next Hendrix to pick up a guitar, it's hard to say what's next for Paul Allen.

He does say this: "I'm not near the end of the story."