IDroo - A shared whiteboard for use by multiple Skype-users simultaneously. Features
skrbl - Simple and easy online multi user whiteboard, start skrbl, give out your URL & start working...
NoteBookCast - A real time shared whiteboard in your browser, teach, explain, sketch. For work, study, fun.
Scribblar - Scribblar is a free online collaboration tool that is perfect for online tutoring.
Twiddla - Mark up websites, graphics, and photos, or start brainstorming on a blank canvas.
CoSketch - 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.