Microsoft has announced it has open sourced the server side elements of .net as well as making parts of .net cross platform.

Server Side Only

Currently, only the server side stack is being opened source and while that includes a lot of functionality, the cross platform client side aspects of .net are still being worked on by Xamarin (in collaboration with Microsoft). Making the WPF run on multiple platforms is an extremely difficult task with diminishing returns given that less and less applications are on the desktop.

Visual Studio Community Edition

There have been free editions of Visual Studio Express for years, and while there are additional functions included such as access to Visual Studio extensions, the community licensed versions cannot be used for enterprise application development. Visual Studio will remain proprietary and I assume a Windows only application.

Will it pay off?

I’m still not sure it is enough to overcome barriers to entice developers to adopt the Microsoft platform. When I think back to when I had to choose between .net and Java, I still see a lot of the same cons despite this announcement. At the time my assessment was based on how limited would I be adopting Java or .net. With java I could branch out in any direction and not hit many limits in terms of cost, support, library availability, or development and target platform. In a nutshell, if I created a start up tomorrow, how far would I get before I had to start shelling out thousands of dollars for software to develop with.
With Java I have free development tools running on free linux running on free application servers using free libraries and free databases on a variety of hardware platforms. Under .net, Visual Studio was limited unless you purchased the full version and many (not all) third party libraries and additional software products were commercial. The server software running the applications are still commercial.

Most of this still applies despite the partial open sourcing of .net, and given the same choice today, I would probably reach the same outcome. While this news is great for the open source proponents of free speech, it does little for the free beer brigade, and the free speech proponents are still likely to be put off by the other closed source aspects of Microsoft. Even Oracle who open sourced Java years ago are still running being accused of not being open source enough.