Scriblink is a site that offers basic whiteboard capabilites, for free, within your web browser. What is really cool about Scriblink is you can write on blank pages, upload your own photos and images, or paste a URL of an online image and add your own annotations to it.
Scriblink allows you to share the whiteboard with a friend via a unique URL that you share with them. There is even a chatbox to chat with those you are sharing your whiteboard with.
Finally, you can save your images you create in Scriblink to your hard drive for later review (students who are absent?). Scirblink offers many awesome features with the price tag we all love, free. This would be a great resource for collaborative mind mapping, organization, working math problems, brainstorming with a partner, and much more. The only downside is a very small banner ad at the bottom of the site, which is not very obtrusive to the experience at all.
Scriblink works fantastic if you need quick whiteboard interactivity in your classroom.
Check it out at: http://scriblink.com/