Pencil - Open Source vectorial drawing/animation !!!
The Pencil application is an Open Source vectorial drawing application, with animation support. It uses the Qt toolkit making it compatible with Windows, OSX and Linux. You can also create bitmap images and the support for “onion skinning” allows you to see previous (and next) frames when creating animations.
As always with this kind of applications, having a Tablet/Pen/Stylus and artistic feeling will improve your results.