Stoodle is a very nifty web tool for creating collaborative whiteboards. It can be used on the iPad or on a desktop/laptop. No registration is necessary, simple start a new class classroom, share the URL with others, and begin collaborating. Microphone support allows for audio conferencing between users, and there is also a chat box available. Type, draw, add images, and more. Stoodle also allows for multiple pages, so you never run out of room to work.