If Danielle Fong were a fictional character, she wouldn't be plausible. She lives in a refurbished fire station, started college at age 12, and has a plan to boost the viability of green energy that has attracted attention from some of the world's greatest thinkers. LightSail's technology represents a new take on energy storage that it says will be cheaper and more efficient -- retaining some 70% of a stored charge, much higher than what's possible with existing equipment. Energy storage is a serious hurdle for power sources such as wind and solar, which can only be harvested for part of the day. If it works, LightSail's solution could "make renewable energy practical and mainstream for the first time," says Vinod Khosla, an early investor in LightSail. He's not the only luminary backing the company. Bill Gates and Peter Thiel are also on board, helping boost the startup's funding to more than $50 million. Fong, who drives a Tesla and uses Google Glass, lives in the firehouse that was her company's original headquarters and says she'd like to throw her 40th birthday party "in space." Far-fetched? Maybe not. -Anne VanderMey
Will Dean & Guy Livingstone
Think you're tough? Nowadays, running one of these events is the best way to prove it. In a fast-growing field of adventure-challenges with names like Spartan Race and Warrior Dash, the Mudder, which Livingstone and Dean came up with when Dean was at HBS, is arguably the best known thanks to effective brand-building and partnerships with big-name sponsors such as Dos Equis (many participants swig a beer after they finish the race), Degree, and Under Armour. With an ever-growing number of events (53 this year, in five countries) and participants (more than 460,000 in 2012, bringing some $75 million in revenue), it's clear these two guys and their business are showing some stamina.-Daniel Roberts
This tech veteran rarely shies from a challenge. After a nine-year stint in Google's ad department, White was recruited by Sheryl Sandberg to head Facebook's sales and marketing for local products. In 2011, she was tasked with monetizing the social network's potentially lucrative but under-tapped mobile platforms, and six months ago, White became the Sheryl to Instagram's Marc Zuckerberg. Now overseeing operations at the company—Facebook bought it for $715 million in April 2012—she's working with Instagram founder and CEO Kevin Systrom to turn the expensive acquisition into a profitable business. First plan? Making the photo-sharing app an advertising platform, without harming the brand's cool factor.-Colleen Leahey
The MIT engineer's waste to energy pilot projects paid off. Bakaya is now operating a commercial plant in Salt Lake City, Utah, working with the state's largest recycler, Rocky Mountain Recycling. The plant can convert 20,000 pounds of non-recycled plastic to 60 barrels of oil per day—all with zero emissions. And she is doing it at a fraction of the cost, which means heady margins: A barrel produced at $25 to $30 at the plant is sold to local refineries for $100. Green energy investors are taking note.-Rupali Arora
Try to imagine a better political resume: a Harvard-trained lawyer, decorated veteran of the wars in Iraq and Afghanistan, and McKinsey consultant who left all that behind to go back to work on the family cattle farm. That background helps explain how this conservative Republican has rocketed onto the national political stage less than a year into his freshman term in the U.S. House — and why he's already jumped into the race for a Senate seat against a two-term incumbent Democrat. Cotton has a good chance to win. He's media-friendly and a fundraising powerhouse, traits that have earned him establishment support as he keeps the faith among right-wing activists. -Tory Newmyer
The 35-year-old runs one of only a dozen uranium-producing companies in the world, and his South Texas operation is the first to go into production in the U.S. in over 7 years. (Barriers to producing uranium—used for nuclear energy—are notoriously high.) Still a junior uranium miner, the NYSE-listed company generated almost $14 million in 2012 revenues, up 62%, and has topped a market cap of $200 million. Blackrock and Vanguard are major shareholders. The spunky entrepreneur has also founded Brazil Resources Inc., a small cap gold producing company in Brazil.-R.A.
Leila Janah is proving to philanthropists and corporations alike that digital jobs can help alleviate global poverty. She founded the nonprofit Samasource in 2008 to link citizens in developing countries with what Janah calls "microwork," small tasks such as image tagging and transcription. The organization has so far lifted 3,800 workers and their families above the poverty line, by winning contracts from the likes of Google, Microsoft, Walmart.com, and Getty Images, and by securing funding from the MasterCard Foundation, the Rockefeller Foundation, and the U.S. State Department, among other institutions. Janah has also launched a crowdfunding site for medical surgeries, Samahope, and is piloting a U.S. work training program, too.-Catherine Dunn
What do Rihanna, President Obama and garage bands have in common? They're all on SoundCloud, the Swedish-born, German-based audio sharing service that's become a go-to destination for music lovers and podcast devotees alike. Founded by Swedes Alexander Ljung and Eric Wahlforss, SoundCloud tripled its registered users since last year, reaching 40 million this summer, and doubled the amount of time those users spend playing tracks, thanks in part to new embed capabilities on Facebook, Twitter and Google+. All told, some 200 million unique visitors head to SoundCloud every month.-Eli Epstein
Alexa von Tobel
Personal finance guru Suze Orman has some competition -- and it comes in the petite, tech-savvy package of von Tobel. After working for 2 years as an analyst at Morgan Stanley, the entrepreneur attended Harvard Business School for a semester before dropping out to launch her company in 2008. LearnVest, which has raised $41 million, targets 35- to 50-year-olds working to maximize their finances. For various prices, its 500,000-plus users can connect with financial planners and tools that help them map out their expenses (and the website's money management platform is free). In 2012, LearnVest became a registered investment advisor with the SEC and this December, von Tobel publishes her advice tome Financially Fearless.-C.L.
Parsons is a hedge fund star in the making. After racking up solid returns over the past decade at Viking Global Investors, the $20 billion fund founded by Julian Robertson's acolyte Andreas Halvorsen, Parsons quit last year over disagreements about how the fund was run. Now he's reportedly opening his own hedge fund by year's end. He'll focus on technology, media and telecommunications stocks and look to become a third-generation Robertson star.-Scott Cendrowski
Maren Kate Donovan
Need help running your life? Zirtual founder and CEO Maren Kate Donovan can assist with that. Or rather, her company can, by pairing clients with vetted virtual assistants who work remotely (even following orders from clients in Antarctica). Donovan, once a top-selling jewelry retailer on eBay, scored $2 million in funding this year from Zappos CEO Tony Hsieh, VegasTechFund, and Mayfield Fund. The investment lured her headquarters from the Bay Area to Las Vegas, where she joined a budding tech scene fueled by Hsieh's ambitious revitalization effort, the Downtown Project. In the entrepreneurial world, Zappos ain't a bad name to have standing behind you.-C.D.