Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License
model - shade - animate - render - composite - interactive 3d

Interface

Fully customizable Interface allows greater range of layout configurations
Non-overlapping and non-blocking UI delivers unsurpassed workflow
Flexible and fully configurable window layout with as many screen setups as you prefer
Undo support on all levels
Anti-aliased fonts with international translation support
Any window space can be easily switched to any window type (curve editor, NLA, 3D view etc)
Built-in text editor for annotations and editing Python scripts
Graphical user interface for Python scripts
Fully scriptable UI with custom themes
Consistent interface across all platforms

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Rigging

Fast skeleton creation mode
Interactive 3D paint for vertex weighting
Fast envelope based skinning
Automatic Skinning that really works (heat equilibrium based)
Mirror editing (bone creation and weight painting)
Double Quaternion deformation reduces shrinking and other bone deformation errors
Volume deformer uses a mesh cage to deform complex meshes with great results
Bone layers and colored groups for better rig organization
B-spline interpolated bones; forget about elbow twists
Constraint stack for IK solver setup and other constraints
PyConstraints; if you need something not yet implemented code it in python with real time feed back, no compiling needed

Animation

Armature (skeleton) deformation with forward/inverse kinematics with pole target support
Auto IK allows posing FK chains easily
Non-linear animation editor for mixing individual actions created in Action editor
Automated walkcycles along paths
Animated constraint system
Vertex key framing for morphing, with controlling sliders
Edit and create new blendshapes from existing targets
Character animation pose editor
‘Ipo’ system integrates both motion curve and traditional key-frame editing
Audio playback, mixing and editing support for sound synchronisation
Timeline offers fast acces to many playback functions, autokey, help markers
Python scripting access for custom and procedural animation effects