DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
PaintBerri - PaintBerri is a free art site that offers an awesome oekaki community.
PaintTool SAI - Learn to draw in different ways.
Brush Ninja - Hand-drawn Animated Gif Creator
Excalidraw - Excalidraw is a whiteboard tool that lets you easily sketch diagrams that have a hand-drawn feel to them.
okekai - A small community site where you can draw, animate, and chat through pictures all for free