Anima2D, a popular skeletal animation software, will be available to the entire Unity developer community for free starting in January 2017.

 Anima2D features:

• 2D Bones
• Sprite to mesh conversion
• SpriteMesh Editor
• Automatic weights
• Weight Editor
• Inverse Kinematics
• Save / Load Poses
• Atlas compatible
• Bake Animation to Bones
• Onion Skin
• Create Avatar Masks

More info: Anima2d guide and in Unity blog