San Francisco-based Dropbox has brought online storage and sharing to the masses. Founded in 2007 by Massachusetts Institute of Technology students Drew Houston and Arash Ferdowsi, Dropbox lets users store their photos, videos, documents and music in an electronic locker that resides in the “cloud.” They can then access or share that information from just about any device that connects to the Internet.

It’s simple. It’s free. And its growth has been explosive. Today Dropbox has more than 50 million users in more than 175 countries. Last October, it raised $250 million from Index Ventures, along with other investors including Goldman Sachs, Benchmark Capital and Greylock Partners, giving the company a $4 billion valuation. Even U2′s Bono and The Edge put in some cash.