Online collaborative drawing, for, e.g. teaching mathematics online
Interactive scratchpad Collusion, Bitpaper (approx USD1/page) is a collective sketch page. desmos is an online collaborative graphing calculator. Blackboard Collaborate is a collaborative whiteboard solution. This is the one my old campus supported. I cannot work out how to access it for ad hoc discussion though, as opposed to for as part of a course. Also, it fails to provide a useful general solution, as it is only available licensed per department for USD9000/year, so normal civilians need not apply.
Other options, A Web Whiteboard, Mural, …
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