GE has designed a free web-based whiteboard you can use without any registration and can invite others to join your session via email or instant messenger.
The service offers more advanced drawing tools than just scribbling around freestyle, too. You can create shapes, type text, create straight lines, change the background colour, and stamp various symbols. It’s crying out for a tablet but is still pretty useful.
I may try to incorporate something like this into a future audit planning meeting, as soon as I’m experienced enough to run one.