Explain Everything - Explain Everything is an easy-to-use design, screencasting, and interactive whiteboard tool that...
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.
Microsoft Whiteboard - The canvas where ideas, content, & people come together.
IDroo - A shared whiteboard for use by multiple Skype-users simultaneously. Features
Conceptboard - Instant Whiteboards for Teams & Projects
Scribblar - Scribblar is a free online collaboration tool that is perfect for online tutoring.