I'm the same with:
PlantUML - quick and simple text representation of diagrams, really easy to iterate on and see changes reflected live.
reveal.js - Quick and simple slide creation, I can reorder slides with ease, tweak wording, don't waste time faffing about with fonts, text box sizing etc. All reflected live in a browser.
There are also some online ones, although I'm not sure how well they work in a multi-editor situation.
Might not be flexible enough for a really open brainstorming, but enough for stuff like figuring out domain concepts.