Spine - Spine is 2D animation software designed specifically for software and game development.
Unity - The multiplatform game creation tools for everyone.
Synfig Studio - Just another WordPress site
Godot Engine - Feature-packed 2D and 3D open source game engine.
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.