While Project Spark is also coming to Xbox 360 and Xbox One, for now, the game about creating games has a beta -- and it's only available for Windows 8.1 users.
There's a sign-up page over here, in case you missed seeing it earlier this year. Beta keys should be going out now, so keep an eye on that inbox of yours. I'm unsure if there's a non-disclosure agreement for participants, but I'm hoping they'll be allowed to share their creations on YouTube. Wouldn't mind seeing what the average player is capable of making.
Here's a recent video in which members of the development team create a puzzle and fighting game within Project Spark. In the right hands, this could be very powerful. I'm reminded of that working calculator someone made in LittleBigPlanet using thousands of parts.