If you've ever thought about making professional-quality animations but decided not to because of the cost of the software, your excuse just got up and walked out the door. For Windows, Mac, and Linux, Blender is a free, open-source 3D rendering program that puts the power of the medium back into the hands of the artist.
Blender features a laundry list of animation essentials that can be broken down into categories: Interface, Modeling, Rigging, Rendering, Animation, UV Unwrapping, Shading, Physics and Particles, Imaging and Compositing, and Real Time 3D/Game Creation. The interface, though loaded with complicated tools, is remarkable for neither obscuring nor interfering with the image being worked on. A brief sampling of tools and functions includes 3D paint for vertex weighing; polygon meshes; 2D and 3D procedural brushes; edge, fur, and hair rendering; textures; and collision simulation; as well as support for around two dozen file formats.
Extras can be added with Python scripting, and since it's open source, many scripts can be easily found for download in the user forums. Blender is not for the faint of heart, but fortunately there's a stack of detailed tutorials available on Blender.org.