Synfig Studio - Just another WordPress site
Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
Spine - Spine is 2D animation software designed specifically for software and game development.
Grafx2 - GrafX2 is a bitmap paint program inspired by the Amiga programs Deluxe Paint and Brilliance.
Animation Paper - Animation Paper is a software package for doing 2D animation.
LibreSprite - Free and open source program to create animated sprites.