Slack - A messaging app for teams who see through the Earth!
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.
Smoothboard Air - Smoothboard Air is a collaborative interactive whiteboard application for iPads and Android devices that revolutionizes interaction in lecture rooms.
IDroo - A shared whiteboard for use by multiple Skype-users simultaneously. Features
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.
Scribblar - Scribblar is a free online collaboration tool that is perfect for online tutoring.