When individual developers think of open source, they think "free." And with good cause: Who in their right mind wouldn't be interested in technology that they can get at no cost and use with few licensing restrictions?

When companies think of open source, these days they think "business agility," a quality they increasingly value above all others in the fast-changing marketplace.

The ability to create new applications quickly, reliably and economically is drawing businesses big and small to open source and emboldening them to use it for ever-larger projects, IT practitioners say.

Which is likely why open source's popularity is booming (with a few holdouts). According to the Forrester Research report "Development Landscape: 2013," 76% of developers have used open-source technology "at some level," says Jeffrey Hammond, a Forrester analyst specializing in application development and delivery.

Here are four key reasons why organizations of all sizes are taking open source seriously.

Open source keeps costs down

Cost savings may be only part of open source's allure, but it's still a big part, no matter what size the organization. "How can Netflix charge as little as $8 per month for its service?" Hammond asks. "Because everything is built on open-source software. They focused on content, not building an operating system or a testing framework."

"It's like using Spring, JBoss or Drupal for content management," Hammond says. "Companies get the 'Lego blocks' for free, so they can spend their time and resources building what they want in particular." Enterprises have always customized packaged software such as ERP applications, except now, with open source, that customization is less expensive.

Open source holdouts Though many developers -- and an increasing number of enterprises -- are bullish on open source these days, not everyone is on board. A Forrester survey from the fourth quarter of 2011 found that developers who declined to use open-source tools -- specifically, integration tools -- primarily shied away because they worried about support (a concern cited by 71% of the respondents) and the lack of technical skills to manage open-source efforts (an issue for 42% of the respondents). Respondents also mentioned concerns about security and licensing to a lesser extent.

Indeed, in some cases, open source is helping to bring back custom development of applications, an option that has decreased in popularity in the past 10 years or so as the use of commercial applications and software-as-a-service offerings gained ground.

Taking on some custom development work in order to save money appealed to Carestream Health. A $2.5 billion provider of dental and medical imaging systems with 8,000 employees, Rochester, N.Y.-based Carestream wanted to consolidate the data from its worldwide manufacturing facilities into a single product life-cycle management (PLM) application to reduce new product development and manufacturing time by routing information more efficiently.

"We wanted consistent management of product-related information across our global company footprint," says David G. Sherburne, director of global R&D effectiveness and engineering IT at Carestream. "With a modern platform in place that could be built upon into the future, we were expecting a 5% productivity gain through the integration of existing point solutions and the elimination of manual process steps."