Microsoft has released the Game Developer Kit (GDK) for developers to use for free. Available on Github, the GDK is the same base development kit used by hundreds of game creators today and was previously only available to approved partners building for the Xbox ecosystem. The GDK contains the common tools, libraries, and documentation needed for developers, it's the future of the Xbox ecosystem across all platforms—PC, cloud, mobile, and console.
Learn more from the documentation and the announcement on the GameStack blog.
Available on Github, but only as a set of closed-source .cab files. Just a warning to those who read “Github” and think “open source”.