Spine - Spine is 2D animation software designed specifically for software and game development.
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.
Synfig Studio - Just another WordPress site
DragonBones - DragonBones is an open source project for 2D skeletal animation.
CrazyTalk Animator - CrazyTalk Animator, the next level of 2D animation. Create actors using any photo or illustration.
Anima2D - Documentation Anima2D is a 2D Skeletal Animation plugin for use with Unity 5.