Spine - Spine is 2D animation software designed specifically for software and game development.
Jpixel - JPixel is an editor for making pixel art animations and tilesets.
Synfig Studio - Just another WordPress site
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.
Adobe Animate - Adobe Animate is a Flash, vector animation software.