Sketchpad for Wave is a gadget for Google Wave that emulates the functionality of MS Paint using HTML5 Canvas and Javascript. Based on CanvasPaint by Christopher Clay which he released to the public domain.
Currently only tested in Firefox 3.5 and confirmed not to work in Safari or Google Chrome (yet). You can check it out in action at the official Sketchpad Google Wave thread. (Google Wave account is required, of course.)