Physics engine for XAML
After briefly playing around with SpriteKit on iOS, and enjoying the built-in Physics Engine, I was looking for something similar on Windows.
I was delighted to find Physics Helper XAML by Andy Beaulieu, which essentially gives your XAML shapes mass, and handles all the events and collisions and gravity you need. It's very cool, and you could easily knock up a few games (featuring upset flying creatures for example) for the Windows Store. In fact, now that it supports universal Windows apps, you could quickly knock up a few games for the Windows Phone store as well!