This has been a general trend for a while now from Microsoft, even before Ballmer left, with several individual .NET frameworks being released as open source. But Microsoft recently announced that they are taking the full .NET server stack open source and planning to support other platforms including Linux and Mac. Also, Visual Studio 2015 will have built in support for iOS and Android development, in partnership with Xamarin, (I don't think this will go so far as a cross platform version of Visual Studio...at least not yet).
One would think that this would mean that Microsoft will be working with the Mono project to leverage the effort that project has already undertaken for other platforms (though it has long suffered from the risk of "patent bombing" from Microsoft). But it's curious that Microsoft's press release doesn't mention the Mono project, nor is there any press release on the Mono project site itself yet. I wouldn't be surprised if they make licensing of patents free on Azure but charge some fee for deployments to other platforms/clouds, which would be both a boon and bane for the Mono project.
However, the announcement did include Xamarin, which is a company that provides cross mobile platform .NET tooling on top of Mono (the CTO was the founder of the Mono project). And Xamarin does have a press release on their site. But Xamarin's revenue model is based on per developer seat licensing, and as mentioned in the press release anything beyond their "starter edition" tooling will require a licensing fee. So it's not likely to be free for many enterprise application situations. (Though, who knows, Microsoft could just buy them at some point and fully incorporate it into their Visual Studio pricing if they want.)
Overall, it's an interesting announcement. But it's not quite as "open source-y" as it tries to present itself. There are still paywalls involved for most real solutions (in many cases, they will be costs justified by ROI, but that's analysis that needs to be done on a case by case basis). And Microsoft doesn't really have much choice if they are truly going to be a "mobile first, cloud first" company, because they don't own 90% market share on either of those platforms. But it's definitely another step in the right direction, even if you do have to be on guard for trojan price gouging and platform lock in.