44. Marissa Mayer VP, Search Products and User Experience Google (GOOG) 2008 rank: 50 Age: 34
When she joined the startup in 1999 as the first woman engineer, Mayer gave it a 2% chance of succeeding. Now she not only directs the look and functionality of the $14 billion search engine, she's also behind more than 100 products, like Google Earth and Google Chrome, the Internet browser and operating system that's challenging Microsoft Windows.
Facebook has grown up under Sandberg, who in her previous job built and oversaw global sales and operations at Google. Facebook works with 83 of the top 100 ad spenders in the U.S. and boasts 250 million users and counting. Facebook director Marc Andreessen has estimated that the privately held company will surpass $500 million in revenue this year.