Spine - Spine is 2D animation software designed specifically for software and game development.
Synfig Studio - Just another WordPress site
Duik - Comprehensive animation and rigging toolset for Adobe After Effects
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.
Limber for After Effects - IK system for After Effects using shape layers as limbs for character animation
Adobe Animate (formerly Flash Professional) - An animation tool that used key frame animation for games and movies.