Spine - Spine is 2D animation software designed specifically for software and game development.
Synfig Studio - Just another WordPress site
Duik - Comprehensive animation and rigging toolset for Adobe After Effects
Live2D Cubism - Live2D is the world's first drawing technology to enable 3D rendering of 2D images.
Limber for After Effects - IK system for After Effects using shape layers as limbs for character animation
AnimeEffects - AnimeEffects is a 2D keyframe animation tool based on deformation of polygon meshes.