Collaborative drawing tools

ExcaliDraw: https://excalidraw.com/
a free/libre webapplication that allows you to draw online, alone or together. It’s more powerful than the BBB whiteboard, and allows vector based image files to be exported, and reimported.
Freemium model: The basic service is free, for 7$/person/month you can store files on their server, organise them in libraries and workspaces and so. As free/libre software you can also run a server yourself (code here).

Diagrams.net https://app.diagrams.net/
previously known as draw.io

Non-free collaborative drawing software

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The latest version of Nextcloud also has an app that integrates Spacedeck, an open source collaborative whiteboard:

https://apps.nextcloud.com/apps/integration_whiteboard