Someone very kindly sent me an invite code to the Project Spark beta yesterday. Tonight I had a quick play. It looks to me like a cross between Disney Infinity, Minecraft and Project Kodu. With some Little Big Planet thrown in. You can design and create your own worlds and populate then with objects and characters. Each object in the game has a "brain" which controls how it behaves in response to particular stimulation.
It looks really good, the world editing tools work well and the programming side is very interesting. Microsoft plans to release it across Windows 8 and Xbox One (and I'd love a copy on the phone too).
There are lots of high quality tutorials and you can share the worlds that you make with other people.