This Internet Whiteboard is called Imagination Cubed. Not only fun, it can also be used for serious things like sharing ideas by drawing, creating mind maps or even website layouts.
The tool turns your mouse into an animated ink pen or brush. You can do freehand drawing or even type on any area of the screen. Your drawing strokes are recorded by the software which you can later replay or share the artistic sketch with friends.
The most impressive feature is "Draw with friends" - You can send an email or IM your friends and they can join you in the same drawing. Excellent while you are doing a brain storming session throught video conferencing on the web.
There's also an Ink meter on the screen - the more you paint or type, the more ink is consumed.
Imagination Cubed can keep you busy and entertained for a long time. Flash Player is required to use this site. It may show some problems with Flash Player 9. There's no way to import existing images or documents.