Spine - Spine is 2D animation software designed specifically for software and game development.
Synfig Studio - Just another WordPress site
Creature - Creature 2D Skeletal and Mesh Animation Tool
Animation Paper - Animation Paper is a software package for doing 2D animation.
Unity 2D - Unity 2D is a powerful and feature-rich game design solution built from the ground up to fulfill every basic and advanced need of the developer.
CrazyTalk Animator - CrazyTalk Animator, the next level of 2D animation. Create actors using any photo or illustration.