Synfig Studio - Just another WordPress site
Spine - Spine is 2D animation software designed specifically for software and game development.
Duik - Comprehensive animation and rigging toolset for Adobe After Effects
Limber for After Effects - IK system for After Effects using shape layers as limbs for character animation
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.