FOr instance, the Zettelkasten methodology is quite frequently cited. However, most note-taking systems I saw so far are either too simplistic (missing support for embedded images, tables, easy linking between notes, ...), are only designed for input but not recall (i.e. plan text, monospace input and one needs to render the output for it to be readable), or only work online (e.g. Roam).
So, I was wondering what HN uses for full-fledged personal knowledge management
Of course adding another SAAS tool just makes it harder to find things, so the disease feeds itself.
I think the first step of this kind of system is being able to suck in data from other places (email, web history, document folders, ...) and search for content. At that point you might get away with just taking notes with whatever text editor or word processor you like (it is very easy to work with docx documents) or build a special note-taking system oriented around "annotate everything".
One 'brain extension' ability I find quite useful are systems that can break work into small chunks and manage their workflow.