Sheryl Sandberg was named by Fortune Magazine as one of the Fifty Most Powerful Women of 2007. At age 38 she is the youngest woman on this list. Sandberg is the Vice President of Global Online Sales and Operations at Google. She helped build the Google advertising empire.
See her speak in the video below about Google's ambitious philanthropy plans.

Update March 4th 2008: Sheryl Sandberg has just left Google to go work at Facebook as their new chief operating officer.