I often do remote video meetings which require writing and discussing math: formulae and plots. I've tested couple virtual whiteboard software applications but they are inconvenient and require using tablets of writing with your mouse. I ended up pointing my camera to a regular whiteboard during the meeting and photographing and emailing content as meeting minutes. Obviously there are more sophisticated solutions like these which cost thousands of dollars. The main problem with such solutions is that you have 2 whiteboards instead of one. Both parties do not draw on the same surface, but each draws on it's own. You could not correct or modify other person's writing. Even more expensive solutions like this, replace whiteboard with huge touchscreen TV solve this problem.
I thought that that this problem could be elegantly solved by augmented reality using Google or Oculus Rift VR glasses. Your correspondent's drawing is virtually projected on your whiteboard and yours is projected on his. While each whiteboard only has physical drawing from one of you, when you look at it through the glasses you see both whiteboard contents superimposed on top of each other. This should work with regular $40 whiteboard and crayons or even with a piece of paper. The solution is very portable. All you need is to carry your glasses and you can project a piece of paper from your hotel room.