Several years ago I worked on an XNA game called "Dangerballs" with a small group of very creative people. My chief problem was our animator would come up with these imaginative animation sequences, which he described to me via sketches and Flash mockups, which I'd then have to implement and tweak in code. It could take a solid weekend to implement a basic 10 second animation.
One thing led to another, and eventually this key-framed animation editor was born.
Essentially it starts with spritesheets, which are chopped up into rectangular regions to create the fundamental animations (support for packed spritesheets was later added). Composite animations are then built by adding one or more child animations, which each get a timeline on which you can create key frames and specify 2D state, like position, rotation, scale, colour, for the engine to lerp between.
Unfortunately Dangerballs never progressed beyond a single level gameplay demo.
However the animation system did become the basis of an animated ebook engine for mobile devices, which was used in a few product pitches.
The application is an unusual combination of .NET winforms, and XNA spritebatch rendering.