PaintBerri - PaintBerri is a free art site that offers an awesome oekaki community.
Spine - Spine is 2D animation software designed specifically for software and game development.
okekai - A small community site where you can draw, animate, and chat through pictures all for free
Synfig Studio - Just another WordPress site
Brush Ninja - Hand-drawn Animated Gif Creator
DragonBones - DragonBones is an open source project for 2D skeletal animation.