Spine - Spine is 2D animation software designed specifically for software and game development.
Synfig Studio - Just another WordPress site
Live2D Cubism - Live2D is the world's first drawing technology to enable 3D rendering of 2D images.
Pixen - Pixen is a professional pixel art editor designed for working with low-resolution raster art, such as those 8-bit sprites found in old-school video games.
Jpixel - JPixel is an editor for making pixel art animations and tilesets.
AnimeEffects - AnimeEffects is a 2D keyframe animation tool based on deformation of polygon meshes.