In a major expansion of its open source efforts, Microsoft announced today that will open up Visual Studio and .NET to developers on all major platforms. It starts with the open-sourcing of the core .NET server stack, a new free and full-featured Visual Studio edition, and preview versions of the next generation versions of Visual Studio and .NET... which can be used to build iOS and Android apps.

This is uncharted territory, folks. But if your knee-jerk reaction to Microsoft putting mobile apps on Android and iOS before Windows was negative, don't let this one keep you up at night. What it means is that Microsoft is finally letting developers use the superior Visual Studio environment and .NET tools and frameworks to create truly cross-platform solutions. In other words, this is good news.

Here's what's happening.

Microsoft is open sourcing the full server-side .NET Core stack. This includes the .NET Framework Libraries, .NET Core Framework Libraries and the RyuJit VM, and it will allow .NET applications to run on both Mac OS X and Linux in addition to Windows.

Visual Studio Community 2013. While I feel that Visual Studio should simply be a single, free product (as are comparable developer environments on OS X and Linux), Microsoft has at least created a new Visual Studio product, called Visual Studio Community 2013, which is "free and fully-featured" (meaning that it supports all Visual Studio extensions, among other things). It's available today and can be used to "target any platform, from devices and desktop to web and cloud services," excluding enterprise applications. Yes, it only runs on Windows, and it appears that you have to remove other Visual Studio versions before installing it. So I'll check it out in a bit.

Visual Studio 2013 Update 4. The latest update to Visual Studio 2013 is now available and includes "dozens of improvements across the product plus several great new features." (Visual Studio Community 2013 includes this update.)

Visual Studio 2015 Preview and .NET 2015 Preview. Previously developed under the name Visual Studio 14, the next version of Visual Studio will be called Visual Studio 2015. Microsoft has provided preview versions of Visual Studio 2015 and .NET 2015 if you want to get started early. This release includes the "Roslyn" .NET compiler platform, new language features in C#, improved debugging and profiling, and code analysis tools. But here's the big deal: It also supports "C#, C++ and HTML/JavaScript development targeting iOS, Android, Windows and more." There's even a Visual Studio Emulator for Android. Yikes!

Visual Studio Online. Microsoft's cloud-hosted development backend is "expanding its DevOps portfolio with the new Visual Studio Online Release Management service and Visual Studio Cloud Deployment Projects," Microsoft says.

So this is big news. As you may recall, Microsoft previously open-sourced a few key components of .NET, including ASP.NET and the C# compiler, but today's announcement is a major expansion with the entire .NET Core server stack going open source. This process will take several months, Microsoft says, and the remainder of the .NET Core Runtime and .NET Core Framework will be released under the MIT open source license.

If you're a developer or otherwise understand what's happening here, you may be wondering how this announcement impact Xamarin, Miguel de Icaza's platform for using Visual Studio to build cross-platform apps that target Android, iOS and Windows Phone. In a move that sort of resembles Microsoft's partnership with Dropbox to me, the firm says it is jointly providing a discount to eligible MSDN subscribers for Xamarin Business or Enterprise, and increasing the integration between Visual Studio and Xamarin, with a streamlined experience for installing Xamarin from Visual Studio. And Xamarin will add Visual Studio support to its free offering Xamarin Starter Edition later this year. This release will "complement" the release of Visual Studio Community, "opening up .NET cross-platform mobile development to any developer."

Again. Big news.