Online Collaborative Whiteboard

This whiteboard - "AWW: A Web Whiteboard" - allows you to draw on a blank slate in your browser. More importantly, it is a collaborative space that students can join from anywhere in the world with a broadband connection. I've found it useful for running a whiteboard concurrently with a Skype tutorial on snow days and after the Chch earthquake.

It's particularly useful if you have a stylus. If you're thinking of using this long-term then I recommend picking up a cheap stylus so that your writing is legible! (I recommend this wireless tablet+stylus for durability in the classroom, and you can take it home to use with this whiteboard after school)

An alternative online whiteboard tool is, try it and see which one you prefer. There are a few others out there but these two are the fastest to get going. If you want a more reliable one with more features there are paid options, and is solid but takes a long time to set up and get going. Add your suggestions in the comments below for how this could be helpful for students' learning.