Pencil2D is lightweight and easy to use so you can focus on animating and not what button to hit next.
Raster & Vector Drawing
Pencil2D allows you to seamlessly switch between raster and vector workflows, allowing you to sketch, ink & paint on the go.
Pencil2D is cross-platform and runs on Windows, Mac OSX and Linux.
The source code of Pencil2D is completely open and the software is free to use even commercially!