Spine - Spine is 2D animation software designed specifically for software and game development.
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
Synfig Studio - Just another WordPress site
Grafx2 - GrafX2 is a bitmap paint program inspired by the Amiga programs Deluxe Paint and Brilliance.
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.
Piskel - Piskel is a website where designers online create sprites or pixel art.