Spine - Spine is 2D animation software designed specifically for software and game development.
Dotpict - Easy and intuitive, mobile pixel art editor.
Synfig Studio - Just another WordPress site
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
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.
Adobe Animate (formerly Flash Professional) - An animation tool that used key frame animation for games and movies.