For an example of a whiteboard that I created, once again for students to collect data collaboratively in a science lab setting, see the url that was provided when I started this whiteboard... http://www.twiddla.com/466048. Students simply need to be given the url and they can begin adding to it - no email addresses or passwords needed! Now, the board is technically public and anyone could access it. But, this also means that students can access it at a later date and a different location as well.
I need to play with Twiddla a bit more to discover its true capabilities, but one cool note about it..... Check out this great feature... "Don't forget that we're still Free for Educational Uses! Sign up for a Free Trial, then send us an email from a .edu address (or whatever screwy extention they use in your land), and we'll hook you with a Pro account free of charge. We're at firstname.lastname@example.org. Be sure to include your username!"