Synfig Studio - Just another WordPress site
Spine - Spine is 2D animation software designed specifically for software and game development.
Duik - Comprehensive animation and rigging toolset for Adobe After Effects
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.
DragonBones - DragonBones is an open source project for 2D skeletal animation.