When Steve Jobs returned to Apple in 1997 after a 12-year exile, the company was close to bankruptcy. Thirteen years later it has a market cap of $250 billion and is the world's most valuable tech company, transforming whole industries along the way. iTunes reinvented music. Pixar, now part of Disney, elevated animated films. The iPhone changed telecom. And the new iPad has other computer makers scrambling to respond. Rocking one industry could be luck, but upending four? That's smart.

Jeff Bezos never stops innovating. Bezos launched Amazon as an online bookstore in 1994, then turned it into a massive virtual mall open to other vendors' wares. Now he's done the same with Amazon's cloud computing services, which let customers rent data storage and computing power from Amazon. Then there's the Kindle, which has helped redefine the very concept of the book. Though the company hasn't disclosed the number of Kindle users, its eBook selection has grown tenfold to 600,000. And while the iPad is getting all the eReader love these days, Bezos got there first, and virtually every iPad review compares Steve Jobs' tablet to Bezos' device.

For the 192,000 men diagnosed with prostate cancer last year, Dr. Mitch Gold's Seattle-based BioTech company Dendreon may hold the key to their survival. The Food and Drug Administration last April finally approved Provenge, a prostate cancer vaccine that stimulates the body's own immune system to attack tumors. In one clinical trial, Provenge extended the lives of patients with prostate cancer on average by 4.1 months, with 32% of patients still alive three years after treatment. While Gold and Dendreon stand to benefit -- it's estimated Provenge could generate more than $1 billion in U.S. sales -- ultimately the real beneficiaries will be the patients themselves.

Like many entrepreneurs, Ma's success story started with borrowed capital and a dream. In 1999, Ma, a former English teacher, invited 18 people over to his home, pitched the idea of a business-to-business e-commerce site, and snagged $60,000 in funding. Today Alibaba's multitiered empire, which includes e-commerce, consumer retail, and payment platforms and ownership of China Yahoo, now plays a major role in China's booming economy.

Rosenblatt's big media idea is simple but brilliant: Give the masses what they want. His Demand Media uses a team of freelancers to churn out 7,000 videos and articles a day, based on a sophisticated computer analysis of what online readers are searching for on any given day. The material appears on Demand's own family of sites, but also on mainstream media outlets such as USA Today. Readers are happy, and advertisers apparently are too: Executives say that Demand Media has been profitable since its start, reportedly generating more than $200 million in revenue a year.