Blender
Blender is an amazing open source animation and rendering program. It includes Python scripting to allow user programmable functionality and effects. The code is still very baseic and very rough as I learn the Python API. I hope to soon include a Python effects framework for creating special animation effects as well as provide some information on aspects of Blender that I have had trouble finding information about (such as figuring out the equations behind the texture mapping input types). So: more to come....