Its still a work in progress, but check out my project http://blotjot.com Its a collaborative whiteboard that works in any browser that supports canvas, including mobile safari on the iPhone and iPad. Runs on node.js + socket.io
It's more of a concept app to draw on Chipotle or Starbucks napkins than a serious endeavor. Some fellow engineers and I were joking about it at lunch one day and I promised to make a kick ass web 2.0 app that did indeed enable you to draw on napkins. Then I added some other backgrounds because it seems that whatever medium you draw on affects your thinking in subtle ways.
So yeah, it's more or less pointless. Google wouldn't even give me AdSense for it. :)
This is pretty neat, but you should make it so that if you click, you create a small dot. Right now it looks like you only render when the mouse moves, but not on the initial mouse down event. (tested with latest safari and chrome).
I've been working on a similar idea with canvas, node.js and socket.io - it's amazing how many people are taking these same technologies and solving similar problems.
Example: http://blotjot.com/heyhn