Jpixel - JPixel is an editor for making pixel art animations and tilesets.
Synfig Studio - Just another WordPress site
Animation Paper - Animation Paper is a software package for doing 2D animation.
DragonBones - DragonBones is an open source project for 2D skeletal animation.
CrazyTalk Animator - CrazyTalk Animator, the next level of 2D animation. Create actors using any photo or illustration.
Spine - Spine is 2D animation software designed specifically for software and game development.