Board800 - Free Collaborative Online Whiteboard

Board800 is a simple service that enables anyone to create a collaborative whiteboard space in seconds. Board800's primary functionality is drawing shapes and creating designs. You can also add text to your Board800 whiteboard. To get started using Board800 all you need to do is visit the homepage, click "start drawing," and then click "create a new session." Once you've started a whiteboarding session you can invite people to join you by sharing the session name/number assigned to your whiteboard.

Applications for Education
Board800 could be used by students to create webs or mind maps together. Students could also use Board800 to create sketches and diagrams for use in presentations or blog posts.

Here are some related items that may be of interest to you:
Scriblink - Your Online Collaborative Whiteboard
Scribblar - Simple, Effective, Creative Collaboration
Postica - Collaborative Sticky Notes