Open source software—software freely shared with the world at large—is an old idea. A guy named Richard Stallman started preaching the gospel in the early '80s, though he called it free software. Linus Torvalds started work on Linux, the enormously successful open source operating system, in 1991, and today, it drives our daily lives—literally. The Android operating system that runs so many Google phones is based on Linux. When you open a phone app like Twitter or Facebook and pull down all those tweets and status updates, you're tapping into massive computer data centers filled with hundreds of Linux machines. Linux is the foundation of the Internet.

If there's one thing we learned in 2015, it's that we shouldn't underestimate the power of open source.

And yet 2015 was the year open source software gained new significance, thanks to Apple and Google and Elon Musk. Now more than ever, even the most powerful tech companies and entrepreneurs are freely sharing the code underlying their latest technologies. They recognize this will accelerate not only the progress of technology as a whole, but their own progress as well. It's altruism with self-interest. And it's how the tech world now works.

"This is not just a turning point, but a tipping point," says Brandon Keepers, the head of open source at GitHub, the online service that sits at the heart of the open source universe.

Apple Opens Up

This year, Apple open sourced the Swift programming language—a big departure from how it operated before. For the most part, Apple kept the code underpinning its previous language, Objective-C, to itself, ensuring that it ran only on Apple devices. By open sourcing Swift, Apple ensures the language can run on any device, including machines based on Linux, Android, and Microsoft Windows.

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Yes, Apple is allowing its language to run on competing devices. But this is what it must do. Thanks in large part to the proliferation of open source software, the modern world no longer runs on a single computing platform the way it did in the '90s, following the rise of Microsoft Windows. If Apple wants to keep pace, it must ensure that its coding tools run everywhere. That's because the world's software developers must build for all the platforms people around the world use. If Apple's tools only work for Apple's platform, developers will be less likely to use them.

Not convinced? Late in 2014, Microsoft came to the same conclusion when it open sourced .NET. For years, .NET was merely a way of building software that ran on Windows. Now that it's open source, the wider software community can ensure that software built with Microsoft's tools runs on Linux and Apple's operating system, too.