Skrbl lets you create an online whiteboard in about the time it takes to read this sentence.
Without so much as registering, you can start a new whiteboard. Doing so creates a URL that you send to friends, colleagues or whomever (though you have to do this manually; Skrbl has no invitation tools to speak of). When they hit the URL, they see what you see and can collaborate on the whiteboard.
Create instant online whiteboards with Skrbl - Lifehacker
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