Twiddla is an online whiteboard collaboration tool that's easy to use, doesn't require any downloads or complicated firewall stuff, and is free for just about anybody.
It's like having a whiteboard skin on top of any web page or uploaded image (or nothing at all -you can use it as a blank canvas). You can mark up this whiteboard layer and interact with the web page at the same time - all in real time, with people from all over the world.
Twiddla is a real-time online collaboration tool. Its beauty is in its simplicity and accessibility: no plug-ins or downloads, no need to work around complex firewalls, no advanced scheduling. It's available 24-7 and the only requirement is a computer with an internet connection and a web browser. It's browser-agnostic, user friendly, and probably more fun than you're supposed to be having at work.