Microsoft's ambitious game-creation tool, Project Spark, is making its debut on Windows 8.1 very soon. In fact, the app is expected to launch at some point today on the Windows Store, though you'll need to request a key from Microsoft if you hope to participate in the beta.
Project Spark is essentially Microsoft's attempt at making developers out of its community of Windows and Xbox users. Expected to launch for PC, Xbox 360 and Xbox One, the beta is making its debut on Windows 8.1, with rather high requirements. Microsoft hasn't yet specified how many testers would be accepted into the beta, though you can request your key on the official Project Spark site.
The project isn't entirely original in concept, but may prove to be the most advanced option yet. Users will have plenty of tools at their disposal within Project Spark, allowing a wide range of customizabiliy, not to mention SmartGlass, Xbox One controller, and keyboard/mouse support. Additionally (and perhaps most impressively), the Kinect will provide a means of capturing voice and motion for facial animations, motion, and so forth, whether you're fleshing out a custom character or a cinematic.
The beta will be making its way to Xbox One in January of next year.