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.
OpenBoard - The OpenBoard software is an interactive white board software.
Conceptboard - Instant Whiteboards for Teams & Projects
Scribblar - Scribblar is a free online collaboration tool that is perfect for online tutoring.
Microsoft Whiteboard - The canvas where ideas, content, & people come together.
Groupboard - Free collaborative online whiteboard, web conferencing and online tutoring software that can be embedded in your own website. Used by many online tutors, schools and universities.