Top 10 Whiteboard tools

Whiteboards are realtime online group drawing apps that allow to  mark up documents, websites, graphics, and photos, or start brainstorming on a blank canvas.
Twiddla is a no-setup, web-based meeting playground. Mark up websites, graphics, and photos, or start brainstorming on a blank canvas. Browse the web with your friends or make that conference call more productive than ever. No plug-ins, downloads, or firewall voodoo - it's all here, ready to go when you are. Browser-agnostic, user-friendly.
Whiteboard designed for team collaboration. We improve the effectiveness of visual collaboration between startup teams, freelancers and their customers, project managers, designers and creative communities located all over the world.
Doceri is a scalable suite of products that allows a teacher or presenter to control a computer's desktop, to control audiovisual technology, and to annotate any document or image. The professional iPad interactive whiteboard and screencast recorder with sophisticated tools for hand-drawn graphics and remote desktop control.
Scribblar is a collaboration tool that is perfect for online tutoring and real-time collaboration and used by thousands of teachers, students and schools.
Groupboard is a set of multi-user java applets including whiteboard, chat, message board, games and voice conferencing which you can place on your web page by simply copying a few lines of HTML code. You can also run Groupboard on your own web server. It can be used for tutoring, distance learning, training, or simply for fun! With the whiteboard you can upload background gif/jpeg images and draw on top of them, and all of the users connected to the board will see the changes in real-time.
Flockmod is a realtime online group drawing app. Draw with your coworkers online. Interactive group drawing for the modern age
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. Anything you paint will show up for all other users in the room in real time. One click to save a sketch as an image for embedding on forums, blogs, etc.
IDroo is an online real-time whiteboard for Windows, Linux, Mac, iOS and Android. Endless canvas for your drawings, learning lessons, brainstorms, you name it... Smooth freehand strokes - just like pencil in notebook or chalk on blackboard. Insert Images, PDF, Word or other documents to the board
Scriblink is a powerful tool that allows you to collaborate in real time on a digital whiteboard where you can sketch, plan, collaborate or just have fun.