Invision - Prototyping and collaboration for design teams
PaintBerri - PaintBerri is a free art site that offers an awesome oekaki community.
Proto.io - Create fully-interactive high-fidelity prototypes in minutes that look and work exactly like your app should. Sign up for a free 15-day trial and get started.
okekai - A small community site where you can draw, animate, and chat through pictures all for free
Zeplin - Collaboration app for UI designers & frontend developers
Brush Ninja - Hand-drawn Animated Gif Creator