Spine - Spine is 2D animation software designed specifically for software and game development.
Synfig Studio - Just another WordPress site
OpenToonz - OpenToonz is an animation program that is a free software version of Toonz, the animation software...
AnimeEffects - AnimeEffects is a 2D keyframe animation tool based on deformation of polygon meshes.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
DragonBones - DragonBones is an open source project for 2D skeletal animation.