At a Boston bus station in early 2007, Drew Houston opened his laptop to finish some work while waiting for his bus. As he turned on his computer, he froze. "I could see my USB drive sitting on my desk at home, which meant I couldn't work," the 28-year-old says. "I sulked for 15 minutes and then, like any self-respecting engineer, I started writing some code. I had no idea what it would eventually become." 

What it became was the foundation for Dropbox, a free cloud-based file-syncing service that allows users to access and share their digital files, photos, and videos from almost any mobile device or computer. Today, Dropbox boasts more than 25 million registered users who save over a million files every five minutes. But that didn't happen overnight.   

By June of 2007, Houston had teamed up with fellow MIT computer science student Arash Ferdowsi to start the company. 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 llanded 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.