As a grad student in computer science at the University of Antwerp in the early 2000s, Dries Buytaert began fiddling with what he thought was a fun side project: an open-source software platform that could support all kinds of websites. He called the system Drupal--Dutch for "droplet"--and figured no more than a dozen people ever would use it.

Buytaert graduated in 2007. But unlike his fellow grads, he didn’t have to look for a job. It turned out that his modest side project had captured enough users and buzz to be the basis of a business.

So Buytaert partnered with Jay Batson, a Boston tech entrepreneur he'd met at a conference, and founded founded Acquia, which provides service and support for websites built on the Drupal content-management system. They set up shop in Burlington, Massachusetts, the better to tap into Boston's deep well of tech talent. Tom Erickson, a former executive at the digital mapping company Tele-Atlas, joined the business as a board member in the first year and now serves as CEO, while Buytaert keeps his focus squarely on the technology.

Five years later, Acquia has clients in 25 countries, and has built websites for clients including Twitter and Intel. It’s appeared on the Inc. 500 list two years in a row, landing at No. 109 in 2013; 2012 revenue hit $45.4 million.) It now has more than 400 employees, offices in Germany, Sweden, Belgium, and Singapore, and investors like Goldman Sachs, Accolade Partners, and Investor Growth Capital.

A more astonishing fact: Drupal now provides the backbone for more than 2 percent of all of the sites on the Internet. For that reason alone, Inc. considered the founders of Acquia are strong candidates for 2013's entrepreneur of the year. 

What makes Drupal so different from other operating systems? It’s a highly customizable content platform that can support thousands of development tools that enable users to create and manage a wide range of features, including content, social media, and e-commerce. 

Buytaert and Erickson both oversee day-to-day operations (Batson left the company last year), with the former providing inspiration and creativity and the latter tough business strategy. Buytaert also spends a lot of time and energy evangelizing for the open-source software movement. Last year, for example, he logged more than 200,000 air miles last year to meet with developers, speak at Drupal conferences, and educate potential customers. “Every cell in my body believes opens source is a better way to build websites and democratize technology,” he says. That passion also makes good business sense, helping Acquia attract ever-larger clients, many of whom are more likely to view open-source platforms with skepticism.

The company’s next big milestone is the mid-2014 rollout of Drupal’s latest iteration, Drupal 8. It’s been in development for three years and promises to include loads of new enterprise features and far greater functionality for mobile sites.

Meanwhile, Acquia is adding customers in lucrative industries such as pharmaceuticals, such as Pfizer, and entertainment; one client is the Grammy Awards, whose website gets 90 percent of annual traffic on one night. Quite a feat for a one-time side project. “I didn’t have a master plan,” Buytaert says. “I figured, I’ll just put it out there--if some people can use it as their experimental platform, great.”

Correction: An earlier version of this article listed the White House as a Acquia client. is a user of Drupal, the content platform that Dries Buytaert developed.