Late to the game.

Check out Creatoon: http://www.creatoon.com/index.php

Development has stopped and there is no support so it has been released for free use.

Its main features are:

* real-time editing previewing,
* automatic in-betweening,
* freedom of graphical style,
* combining bitmaps with vector graphics.
but...
CreaToon is not aiming at the traditional animaton market, where animators expect to paint their content directly within the software. In CreaToon, content has to be created separately (scanned in, using a paint program, ...), imported into CreaToon, and then animated within CreaToon.
However, CreaToon does include basic curve drawing tools ("sketch objects") that allow you to draw vector graphics within the program itself.
-Rob A>