Pencil2D
Pencil2D is an animation software for both bitmap and vector graphics. It is free, multi-platform, and open source.
Pencil2D Animation

Pencil2D is an animation/drawing software for Mac OS X, Windows, and Linux. It lets you create traditional hand-drawn animation (cartoon) using both bitmap and vector graphics. Pencil is free and open source.

Download

Wanna try the bleeding edge version of Pencil2D? Download the nightly builds!

Links

Source code

Pencil2D v0.5.4 beta

2013-07-26

Pencil2D v0.5.3 beta

2013-06-28

Pencil Animation v0.5 beta (Morevna Branch) - September 15th 2010

Changes by Konstantin Dmitriev:

Changes by davidefa: