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...
Unity 2D - Unity 2D is a powerful and feature-rich game design solution built from the ground up to fulfill every basic and advanced need of the developer.
Blender - Blender is the open source, cross platform suite of tools for 3D creation.
Adobe Animate (formerly Flash Professional) - An animation tool that used key frame animation for games and movies.