Art of Illusion is a free, Open Source 3D modeling and rendering studio.

Many of Art of Illusion's capabilities rival those found in expensive commercial programs. Highlights include subdivision surface based modeling tools, skeleton based animation, and a graphical language for designing procedural textures and materials.

Full documentation is available online.

Art of Illusion Features:

Primitive and Mesh-Based Modeling
Image-Based Surface Texturing
Procedural Surface Texturing
Bone Animation
Pose/Morph Animation
Integrated RayTracer Suite
Simple interface - only controls relevant to the current task are accessible
Extensible Plugin API