At their E3 2013 conference, Microsoft announced and demoed Project Spark, a game-creation tool for Xbox 360, Xbox One and Windows 8. It uses Kinect and SmartGlass to allow players to build levels and potentially entire games.
Microsoft has revealed to Siliconera that Project Spark will be available free of charge when it is released (though no date has been given). Everything seen in the E3 demo can be created using only free content, though the statement seems to imply that paid content may be released in the future.
Naturally, once you have completed a level, you can upload it and share it with the world, to be played by other Xbox and PC gamers.