draw.io

I use Draw.io for all of my diagrams, brain dumps and idea organisation. You really don't need a paid tool (e.g. Visio) for creating diagrams when you have Draw.io. And I'm sure I don't even use it to it's full capabilities. It's my go to when I need to get something out of my head and onto paper, and the library of shapes and assets is invaluable. And it's forever free. Highly recommend.