In a major strategy change designed to expand its horizons in the cloud, Microsoft will take its key software development technologies into areas that the company has long considered enemy territory — giving developers new ways to use .NET and Visual Studio to make software not just for Windows but also for Linux, Mac OS X, iOS and Android.

The landmark moves, announced this morning, include a plan to open-source the .NET core server runtime and framework, making it possible for outsiders to access and contribute to the code that powers Microsoft’s software development platform.

As part of the change, Microsoft will give developers the ability to use the .NET runtime and framework to make server- and cloud-based applications for Linux and Mac.

Microsoft is also releasing a new, full-featured version of Visual Studio 2013 that will be available at no cost to independent developers, students, small companies and others not making enterprise applications.

And the company is releasing a preview of Visual Studio 2015 and .NET 2015 with new features for building applications that run on platforms including Windows, Linux, iOS and Android.

Taken together, the moves are a massive bet by the company, intended to make Microsoft’s Azure cloud platform more competitive by significantly increasing the flexibility of the company’s developer technologies.

Microsoft counts more than 6 million developers using .NET, but in a world where they have options including Java, PhP, Node.js, Ruby on Rails and many more, the company wants to ensure its long-term viability against the likes of Amazon, Google and other major players in cloud computing.

“We want to build a set of developer tools, developer services, and a cloud platform that is highly relevant for every developer,” said S. “Soma” Somasegar, the vice president in charge of Microsoft’s Developer Division. “Every developer — I would underscore that.”

The new capabilities are part of a broader effort by Microsoft to look beyond Windows to make its technologies work across a wide variety of competing platforms. This has been a consistent theme at the company since Satya Nadella took the reins as CEO earlier this year — prioritizing mobile and cloud technologies over the company’s traditional Windows and Office businesses.

Just last week, Microsoft made its flagship Office apps free on iPads and Android tablets, after previously requiring a paid subscription.

Today’s moves “will shock a lot of folks,” predicted Jeffrey Hammond, vice president and principal analyst with Forrester Research, describing the initiative as “a very different approach than they might have taken under the previous CEO.”

Longtime Microsoft CEO Steve Ballmer’s famous refrain was, “developers, developers, developers!” but he also loved to follow that up with, “Windows, Windows, Windows!”

Microsoft .NET was released 12 years ago, at a time when Windows dominated the computing landscape, but the rise of tablets, smartphones and cloud computing have completely upended the company’s business. .NET has long been the dedicated development framework and runtime for Windows, with 1.8 billion installations of .NET, by Microsoft’s count.

“Microsoft is changing to a company that is less about Windows and more about Azure,” Forrester’s Hammond said. Nadella, who led Microsoft Azure during Ballmer’s tenure, is trying to give new Azure leader Scott Guthrie and his group “all the tools they need to be successful,” Hammond said.

If you really want to be a productivity company, you’d better think about how you’re going to deliver those productivity experiences to everybody on this planet. The company says it will be working closely on the efforts with the open-source Mono community and startup Xamarin, which have previously enabled some cross-platform capabilities for .NET developers. For example, Microsoft and Xamarin will enable a streamlined experience for installing Xamarin from Visual Studio, and add Visual Studio support to Xamarin Starter Edition.

The company says the full server-side .NET Core stack will be open-sourced, including ASP.NET 5, the Common Language Runtime and Base Class Libraries. The open-source .NET projects will be part of the .NET Foundation, with an associated GitHub repository.

“If you are a .NET developer, the opportunities are that much bigger for you,” Somasegar said. “You no longer have to be constrained by any one runtime environment. For the ecosystem, this is huge because we truly are open-sourcing the whole stack and not just bits and pieces of the stack. We have an opportunity to build a very broad, rich, open ecosystem around the .NET technology stack.”

Somasegar said the changes for .NET and Visual Studio reflect the larger strategic direction from Nadella.

“Satya has been super-clear,” he said. “We are the productivity and platform company. You’ve got the mobile platform and the cloud platform, and then there are multiple technology choices that people have to make. If you really want to be a productivity company, you’d better think about how you’re going to deliver those productivity experiences to everybody on this planet.”

Update: Jim Zemlin, executive director of the Linux Foundation, writes in this blog post …

“We do not agree with everything Microsoft does and certainly many open source projects compete directly with Microsoft products. However, the new Microsoft we are seeing today is certainly a different organization when it comes to open source. “Microsoft understands that today’s computing markets have changed and companies cannot go it alone the way they once did. Open source has fundamentally altered the software industry and that puts developers, developers, developers in charge.”

Note: Reference to Xamarin corrected since original post.