Twiddla is a free, no-setup, web-based meeting playground. Mark up websites, graphics, and photos, or start brainstorming on a blank canvas. Voice and Text chat too!
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.
J. Carroll, G. Chin, M. Rosson, и D. Neale. DIS '00: Proceedings of the conference on Designing interactive systems, стр. 239--251. New York, NY, USA, ACM Press, (2000)