K-3D is free-as-in-freedom 3D modeling and animation software.
Read More about K-3D.....

Pencil is an animation/drawing software for Mac OS X, Windows, and Linux.
Read More about Pencil.....

Art of Illusion is a free, open source 3D modelling and rendering studio.
Read More about Art of Illusion.....

Blender is an open source software package for 3D modeling, animation, rendering, post-production, and gaming.
Read More about Blender.....

Synfig is a powerful, industrial-strength vector-based open-source 2D animation software package, designed from the ground-up for producing feature-film quality animation with fewer people and resources.
Read More about Synfig.....