Online collaborative drawing, for, e.g. doing mathematics online or general diagrams. Many tools claim to do this, but I have not managed to have a truly satisfactory experience yet, although I have not much committed time to it. I was hoping for a slam-dunk best option. I suspect that most of the options are going to need some kind of stylus input to be any use, at least for the writing-heavy workflows I care about. So, something that involves a Wacom or an iPad or whatever.
Here are some candidates.
Virtual whiteboard for sketching hand-drawn like diagrams. Collaborative and end-to-end encrypted.
There is a version is with extra work teams features called Excalidraw+ (USD7/person/month).
FigJam is made by Figma who have a very profitable business based on being pretty good at real-time collaboration, so maybe that is worth trying.
Miscellaneou online whiteboards
There are many others.
There are other options which are offered only on assinine enterprise pricing (e.g. Blackboard Collaborate) which I will not trouble with here.
POint a camera at a physical whiteboard
Other deliberation tools
Sometimes we want something that is a little more structured than just a diagram editor, incorporating brainstorming or voting tools