An example script demonstrating how to integrate jQuery and the SVG jQuery plugin with Greasemonkey.
You can draw all over any webpage you want!
Sure, right on this page actually if you're using Firefox. Just press alt-w and start drawing. Then alt-w again to turn it off.
First you'll need to download Greasemonkey if you don't have it already.
Then the script itself can be downloaded from here. Same controls as before, alt-w to turn it on and off, or modify the script if you want a different key combination.