Collaborative animated painting, with stencils
As usual, this is a shared canvas. Everyone who plays with this applet is collaborating to create one animation.
If you just use the black and white brushes in the palette on the left, you can make animations by painting every frame.
But if you use the stencil tool (top left), you can highlight a region of the canvas, and that will become the "shape" of your brush. If you then select the black or white brush, your selected shape will be painted onto the canvas in black or in white.
You can also try this neat trick: After you've created a stencil shape, and have selected the black or white brush, hit the play button (on the lower right) to run the animation while you are painting. Now you can paint a moving object into your animation.
To get rid of the stencil (because you just want to paint individual black or white squares), click on the stencil tool and then click on either the white or black brush.
Also, you can use the ← and → keys to step through the animation frame by frame. When you do this, you'll see a shadow of the frame you were just on, which makes it easier to create frame-by-frame animations.