Jpixel - JPixel is an editor for making pixel art animations and tilesets.
Synfig Studio - Just another WordPress site
Duik - Comprehensive animation and rigging toolset for Adobe After Effects
DragonBones - DragonBones is an open source project for 2D skeletal animation.
Limber for After Effects - IK system for After Effects using shape layers as limbs for character animation
Spine - Spine is 2D animation software designed specifically for software and game development.