I'm continually baffled at how the rest of the world hasn't discovered this, but it could be because I sort of live at the margins of "ultra geek" and "normie."
Local, self-hosted "wiki-like" tool. You write notes, it saves them, supports links and todos and calendars etc. Tons of plugins for math, git, etc etc.
The killer aspect, I think, though. Any hypothetical "grandma" can open it up and use it immediately and it's very useful. But because it saves as plain-text (Markdown-ish) with links and pages equalling files and folders, it's always infinitely extensible with Bash or other scripts. For me, it's all the extensibility of org-mode without, I mean, you know.