A paper notebook small enough to fit in my jacket pocket, and so that I can carry anywhere.
I can cross things out, I don't have to futz with software worrying about making it look pretty.
I can scrawl, and draw crudely the visions that I'm trying to capture, again without worrying about how exactly to use the software tools.
If I'm having trouble getting started, I start drawing circles on a page, anything to get my pen moving.
If I want to move a paragraph or sentence somewhere else, I circle it, and draw an arrow to where I want it to go.
Editing and putting it into a word processor comes later.
Just get the damned words out of my head.
I am using LibreOffice these days for a word processing.