Twiddla is an free online collaborative whiteboard – with no account creation required!
The downside is that the URL is public and only temporary… and you cannot save your work. However, all computers are equipped with a screenshot functionality so this should not be an issue. One could also easily call the people they are working with via Skype so everyone could discuss what they are thinking throughout the process (the Twiddla audio functionality requires an account – although there is a free 30 day trial version of it as well).
I don’t see this software getting much use in a traditional classroom, however, it’s possibilities in teacher’s college seem promising for group work when the group can’t meet. It’s a great way to mark up documents, images, and webpages collaboratively.