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Baiboard is a free iOS Whiteboard app that allows you to add text, speech bubbles, highlight, snapshots, web sharing, and much more. The chat feature allows you to record your voice and post it to the canvas. There's no sign up necessary and students can create a board in seconds and even password protect it.
This app is useful for working collaboratively on a project or responding to a prompt. Not only does it get them to have to work together but more importantly, it helps them to create some working norms within their group, sort of a self-policing mini-classroom.