Obsidian, featured in the meme, is on Linux, and for a while now free of charge (but with paid official plugins for syncing and web publishing), and I'd say is pretty good.

I honestly just take notes in sublime text myself, which is paid, but I'm paying for it anyways and often have it open in the background. It can store unsaved tabs in some session file, so I can just put unimportant notes in a tab and have them available to open later.

I also want to mention nextcloud notes, I don't use it for much so I don't know how good it is, but I have a nextcloud instance and use the notes for simple notes I want to be able to access from my phone.

I find that the default gui editor that comes with each desktop is good enough in most cases (xed with cinnamon, mouse with xfce, kate with kde, etc). But if you're looking for a terminal editor, a lot of people are praising micro as very user-friendly with solid functionality.

I'm a big fan of triliumnotes.org.

Single application, or if you want to wire it up as a server you can do that as well. Few small quark's - the biggest two are a mediocre mobile experience (to me) and lack of multi user support.