I also use OneNote on a regular basis, but mostly for making notes and scrabbling instead of using paper. I never thought of using it to edit and audit Anaplan Formulas. I will give it a go!
Probably this does not come as a surprise to anybody, but I usually rely on Notepad ++ or Excel (the latter mostly when downloading the whole list of Line Items in the model) for the purposes that you mentioned.
As most of you, I use mainly A+ for Anaplan, Notepad++ and Excel (however, OneNote isn't my thing).
Notepad++ is really useful when you add Anaplan language (apart from that, I use it for JSON from Anaplan API and simple text operations, like sorting, removing duplicates etc.), and Excel is must-have not only for Imports/Exports, but also for things like model analysis using exports from Models→Line items or Lists, or quick JSON→CSV translation.
Actually now that @JaredDolich shared his great list, it reminded me about two useful extensions. Since I assume most of the people are using Google Chrome, please consider these two extensions:
Page Marker - you can easily draw, make circles or whatever you wish on your screen and save the screenshot immediately. I also find it very useful when sharing my screen and presenting that I can draw and mark something on my screen.
Vidyard - instead of typing an explanation of how to do something in Anaplan, I would rather make a short video of my screen and share the link with the client, so they can open it and follow the steps.