DrawPile - Drawpile is a drawing program that lets you share the canvas with other users in real time.
DeviantArt - deviantART was created to entertain, inspire, and empower the artist in all of us.
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.
pixiv - pixiv is an illustration community service where you can post and enjoy creative work. A large variety of work is uploaded, and user-organized contests are frequently held as well.