Draw across the net with other people, play the drawings like animation and also save them for other people on Open Studio. I think this site has been here for at least 10 years, theres tons of interactive stuff on there. Click everything on open studio, some stuff is sort of hidden.
Give your drawings weight and moving aroundness with MOOVL. I like to make a face or animal with separate components on a background and erase the background so the facial features or animal parts sliiiiide down to the bottom of the screen.
I just looked on the net for more. GE seems to have one theres also Draw Ball, which I could not get to work. On the GE one, you can draw across the net with a friend you invite by IM, or give the URL to continue the drawing, but there is a limited amount of ink provided.