Ah! One of my favorite topics!<p>Inkboard was a fork of inkscape that was an experimental collaborative whiteboard. It uses XMPP, and some pieces of it were merged into Inkscape proper well after the fact.<p>Coccinella is a perl/tk IM client that uses XMPP to do whiteboarding.
I haven't used Vyew in a while, but found it useful. Seems to fit your criteria for free, web-based, and real-time.<p><a href="http://vyew.com" rel="nofollow">http://vyew.com</a>