Spine - Spine is 2D animation software designed specifically for software and game development.
PaintBerri - PaintBerri is a free art site that offers an awesome oekaki community.
Synfig Studio - Just another WordPress site
okekai - A small community site where you can draw, animate, and chat through pictures all for free
Unity 2D - Unity 2D is a powerful and feature-rich game design solution built from the ground up to fulfill every basic and advanced need of the developer.
Brush Ninja - Hand-drawn Animated Gif Creator