Free 3D animation package these days it's something very rare if you'll ask me, but Blender exists on the market, well, many years now and makes success step by step by getting new artists and users involved.

Blender is available for many platforms including MacOS and you can download it for free here: http://www.blender.org/download/get-blender/

More about Blender 2.5 roadmap: http://www.blender.org/development/current-projects/blender-25-project/

CGenie interviewed Ton Roosendaal, Chairman of the Blender Foundation and you can read this interesting article here: 

http://www.cgenie.com/articles/130-blending-in-with-the-competition.html