Microsoft Research Kodu Game Lab (formerly called Boku)
Kodu from Microsoft Research is a new visual programming language made specifically for creating games. It is designed to be accessible for children and enjoyable for anyone. The programming environment runs on the XBox, allowing rapid design iteration using only a game controller for input.
George Birbilis @zoomicon 
Now called Kodu Game Lab, available for Windows too for free (supporting the XBOX controller on Windows too, but works with kbd/mouse too).
On XBOX it’s at Indie games section – unfortunately the Greek XBOX marketplace doesn’t have Indie games section yet (wonder why).