What are people using for whiteboarding/diagramming? Online real-time collaboration would be a big plus but if there's an excellent offline diagramming tool that would be worth exploring too.

Screensharing and omnigraffle atm, works well enough

Is there such a thing as a portable diagram data format/standard? I'd hate to put work into a bunch of diagrams only to have the associated application become unauthorized by my company or have the diagramming startup company go under or be acquired rendering my diagrams uneditable.

https://www.youtube.com/watch?v=HmHOYkTVxIg is an interesting talk in this area

https://c4model.com/

and graphviz / .dot files maybe as a format

http://draw.io also has been mentioned a couple of times (now redirected to https://app.diagrams.net/)

I'm a fan of http://plantuml.com (a java program which consumes text and produces diagrams) - particularly for https://plantuml.com/sequence-diagram .