Dabbleboard is a virtual whiteboard that allows you to create, collaborate and share. It has an excellent interface that is easy to use. You can use it for presentations, homework and study sessions, chat, and as a whiteboard.
This could be very useful to teachers and students as a way to collaborate and share. There is a free version and then paid plans if you need more functionality.
Scribblar - online whiteboard
Twiddla - free online team whiteboarding and collaborating
Here is the interface layout:
Here are some examples of what can be done with Dabbleboard: