Spine - Spine is 2D animation software designed specifically for software and game development.
Synfig Studio - Just another WordPress site
Animation Paper - Animation Paper is a software package for doing 2D animation.
AnimeEffects - AnimeEffects is a 2D keyframe animation tool based on deformation of polygon meshes.
CrazyTalk Animator - CrazyTalk Animator, the next level of 2D animation. Create actors using any photo or illustration.
DragonBones - DragonBones is an open source project for 2D skeletal animation.