Some of the world’s wealthiest are going green, spending their own greenbacks to protect the environment and fight global warming. In honor of the 37th annual Earth Day, Forbes are highlighting their picks for the 11 greenest billionaires. These moguls have made significant commitments to the environment, whether through investment in technology, commitment to earth-friendly living or simply by raising the world’s environmental awareness.

Among the green billionaires are high-profile folks like Microsoft (US:MSFT) co-founders Bill Gates and Paul Allen. Gates’ investment firm invested US$84 million in California’s Pacific Ethanol (US:PEIX), which makes ethanol from corn; Allen’s real estate investment company is constructing 100 million square feet of housing made from environmentally sensitive materials in downtown Seattle. While solar energy isn’t viable in rainy Seattle, Microsoft employees also enjoy free access to mass transit and company-subsidized hybrid cars.

But in conservation, Google (US:GOOG) appears to be racing ahead of Microsoft. Google co-founders Sergey Brin and Larry Page, turned their Mountain View, Calif., headquarters into one of the world’s most eco-friendly.

The 500,000-square-foot facility is heated by 9,212 solar panels; all the furniture in one building is made from recycled material, including old blue jeans as wall insulation. Employees can dine (for free) at Café 150, which uses ingredients only from farms within 150 miles of the kitchen. Brin and Page both drive hybrid Toyota Priuses and have made personal investments in Tesla, an electric car maker. And Brin reportedly has a solar-paneled rucksack to power his phone and MP3 player.

America’s largest landowner Ted Turner and eBay’s (US:EBAY) former president Jeff Skoll have used their fortunes to influence eco-culture. Turner was the creative force behind popular TBS children’s series Captain Planet. Skoll was the executive producer of An Inconvenient Truth, Al Gore’s Oscar-winning documentary on the fearsome ramifications of global warming. A portion of the film’s proceeds are being donated to the Alliance for Climate Protection, a nonpartisan research group.

Still, capitalists will be capitalists and it comes as no surprise that some billionaires have ulterior motives for going green. Silicon Valley venture capitalist John Doerr and his firm Kleiner Perkins Caufield & Byers have invested US$200 million-plus in more than a dozen alternative energy companies. He has famously been quoted as saying that “the field of greentech could be the economic opportunity of the 21st century.”

In September, Richard Branson, whose airline business has been slammed by rising fuel costs, announced plans to commit a portion of the profits (an estimated US$3 billion over 10 years) from his transportation divisions to developing renewable energy alternatives.

The US$38 billion (estimated global investment) alternative energy industry has also coined a few billionaires of its own. Zhengrong Shi, chairman of China’s solar energy giant Suntech Power (US:STP) and Tulsi Tanti of India’s wind company Suzlon Energy have both netted billions in recent years by harvesting nature’s own power sources. This year also saw the debut of the world’s first ethanol tycoon, Brazilian billionaire Rubens Ometto Silveira Mello, who has made a fortune distilling processed sugarcane into the clean-burning fuel.