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.
Synfig Studio - Just another WordPress site
Piskel - Piskel is a website where designers online create sprites or pixel art.
Jpixel - JPixel is an editor for making pixel art animations and tilesets.