Microsoft® has just put a new game development platform out in Beta version: SPARK®. This is a game developer’s free form design tool and game engine. The results can be played on Xbox One, Win 8 and soon other platforms.[youtube 8jDY-6aZSjM]
You can read more at TechCrunch: Microsoft’s Project Spark DIY Game Creation Software Beta Begins Today.
Unfortunately this is a Windows 8 app only. As yet it does not even run on RT. It also is more of a novice game maker’s tool. While free form it is still within a set world, a pre-made world that one can modify.
I think The Verge’s article better describes SPARK: Microsoft’s ‘Project Spark’ beta arrives on Windows 8.1, lets you create an Xbox One game.