In 2007, Drew Houston teamed up with fellow MIT computer science student Arash Ferdowsi to start Dropbox. The two hit the ground running. Though Houston had recently graduated, Ferdowsi dropped out of school his last semester to fully commit to the project. The pair worked tirelessly for three months in a cramped office in Cambridge, waking up everyday at noon and working till dawn the next day.

"I think we started like most of these tech companies begin. Just a couple of guys in their boxers coding in a dark room," Houston says. "We just kept our heads down and built."

By September, the team moved Dropbox to San Francisco. The company landed a total of $7.2 million in funding from Sequoia Capital, Accel Partners, Y Combinator and a handful of individual investors, which Houston says was an important milestone for the team. But it wasn't until an instructional video on how to use Dropbox went viral on Digg that he realized the potential of the start-up.

"At the time, Dropbox was closed. After the video went viral, our beta waiting list went from 5,000 people to 75,000 in a few hours. We got a bunch of press and it was clear that we were working on something that people were interested in," Houston says.

Today, Houston and Ferdowsi have 50 employees. The company does little advertising, but instead uses a referral program that gives users free space for sending new customers to the site.

Though the founders won't reveal specific numbers, Dropbox's revenue comes from their premium plans. The service is free for up to 2 GB, of storage, but users can purchase 50 GB of space for $9.99 per month or 100 GB for $19.99 per month.

Users include individuals, small businesses, event-organizers, and even large companies like Red Bull. With half of their users living overseas, the company just launched the service in four more languages: Spanish, French, German and Japanese.

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