Much of the software that powers the world's largest companies, protects our personal data, or encrypts national security information is open to the public. Anyone can download the source code behind Facebook's user interface, Google's Android operating system, or even Goldman Sachs's data modeling program, and use it as a building block for a totally new project.

What's more, lots of this software is actually developed collaboratively, created and maintained by an army of thousands, from unpaid volunteers to employees at competing tech companies.

This is the world of open-source software, where code is written and distributed freely. So how did a business model that essentially revolves around giving away information and products take over the world?