Moonbase

As-Rigid-As-Possible Shape Manipulation is the paper which introduced the technique used by Adobe for the Puppet Tool in After Effects. It is a significantly better approach for shape tweening than that taken by e.g. Flash (or even Synfig), and I think (in combination with inverse kinematics for the manipulation of the control points) it is the future of 2d animation.

(There is a web site with a demo video here)