I found this website really interesting, it's effectively a whiteboard on the web that you can share with and collaborate with other internet users.

Thinkature is a very simple site to use and would be great for online collaboration, and working through issues that need to be put on paper. I reckon it would be a great little tool for Uni students and the like - you can have multiple worksheets, draw lines, arrows, insert images, text boxes and then you can leave it and come back to it and it is all still there.

The screen shot of a demo worksheet is below: