It's called app builder.

http://www.windowsphone.com/en-US/ap...7-5b874fba0888

Basically, it's a prototyping app. What really blew me away is the pre built examples, they look absolutely gorgeous, vibrant, and bold. It really does show metro at it's best, so much better than the completely black ugliness of apps like whatsapp. I've been playing around with the app and it's not too difficult to get the hang of