Twiddla is an easy to use collaborative whiteboard service, but it's really got the potential to be much more. The whiteboard lets you mark up websites and graphics, and even use a drag-drop library of interface elements for working on interface concepts collaboratively. You communicate with others in the meeting with the chat window, and you can also use built in audio-conferencing capabilities. Very impressive, and free. Would make sense for there to be options for private meetings as well as embedding the whiteboard into your CMS or intranet.
I just gave it a try and the images they've provided are very restrictive. They're basically just graphics you drag in, but they can't be scaled outside of their original dimensions properly since they're bitmaps. It's also not a true prototyping environment like Balsamiq or iPlotz, since there are no properties for the elements. The best you can do is draw or even upload your own bitmap elements. It's truly only a whiteboard. That said, the drawing capabilities are decent.
To give it a try, click the Twiddla button: 