CoSketch is a multi-user online whiteboard designed to give you the ability to quickly visualize and share your ideas as images. No registration or plugins required.
M. Suwa, and B. Tversky. Design studies, 18 (4):
385-403(1997)"sketches serve as a 'perceptual interface' through which one can discover non-visual functional relations underlying the visual features." p 401.
D. Carraher, A. Schliemann, and B. Brizuela. Proceedings of the XXV Conference of the International Group for the Psychology of Mathematics Education, (2001)