Linux

Joplin has multiple sync options. Other than Nextcloud, you can use OneDrive, Dropbox, and they have a subscription service as well.

Obsidian is another to look at, but you need to either pay for sync or bring your own sync (~~though I don't know that you can sync to mobile without using their sync~~ edit: see below comment by @fossisfun@lemmy.ml that explains how to do it).

I think it's probably helpful to know if sync across platforms is important to you, and if so, whether you're willing to pay for it. I'm not sure that there are really many alternatives to an app provided for free with free syncing - that costs money to provide and honestly I'd be a bit dubious about using a service like that.

Obsidian is a brilliant note app, it's free provided you setup your cloud sync solution (I use syncthing) as it just creates .md files that you can do whatever you want with

I just sync a directory called "Notes" everywhere and use whatever text editor is most comfortable on any given platform to edit Markdown documents. Helix on desktop, Markor (or, increasingly, Simple Text Editor) on Android. For checklists, same thing except I use todo.txt for the file format, and the todo script on the desktop and Simpletask on Android.

I have been looking for a self-hosted, concurrent collaborative web editor, as asking my wife to write Markdown is a bit much, and the syncing becomes more complex, but I haven't settled on something.

Maybe Notesnook? There's a free version and they also offer a payed plan if you need more options.

For note taking, Joplin is pretty good, in fact I went from Simplenote to Evernote (over kill and not free) to Joplin using Dropbox for the syncing (syncing is done by Joplin, so you don't need a 'syncing' app) between my PCs, all Linux and my Android phone. Simple to set up and free. I do not use it as a Journal as that is extremely poor, but for notes, it's perfect

Just sync .text or .md files using syncthing. Total control!

You don't need your own server to use Joplin. You can select a local directory to store your notes and sync this directory with Syncthing between devices.

I am not sure if this works with iOS though.

Joplin + Syncthing is great, as other people suggested.
I keep it even more simple: I write stuff in markdown, share my .md files with Syncthing and edit them with markdown editors.
The best I've tried so far for Android is Markor, while on Linux I either use Ghostwriter (on KDE) or Marker (on Gnome)

Obsidian and a shared Dropbox/Google Drive/OneDrive folder?

Notesnook and Standard Notes are really good. Both are end-to-end encrypted and FOSS.

Every other app and solution I've tried is kinda janky, e.g. using Dropbox, git, Syntching or some other app to sync across devices. I want an all-in-one, encrypted, cloud-based, FOSS solution.

I've been using Standard Notes for some years now and I'm pretty happy with it.

Been using Joplin for ages, absolutely no dramas with it at all.

TiddlyWiki — a non-linear personal web notebook

TiddlyWiki, a unique non-linear notebook for capturing, organising and sharing complex information Use it to keep your to-do list, to plan an essay or novel, or to organise your wedding. Record every thought that crosses your brain, or build a flexible and responsive website.

TiddlyWiki lets you choose where to keep your data, guaranteeing that in the decades to come you will still be able to use the notes you take today.

https://tiddlywiki.com/

@OrkneyKomodo
I use #syncthing as an alternative to cloud services. Share two folders, no matter where they are, on what device etc, provided the device can run a version of syncthing.
And its not only notes: pictures, movies and whatnot.
I transfered 60gb of pictures from the family oc to my phone with that

Agree about Joplin. No need for a full NextCloud instance, I use the WebDAV option which Apache has pretty much out of the box.

I use Trilium

If you want it to be truly multiplat and want to control it, you either need a self-hosted web service (simple as a basic wiki or as complex as nextcloud) or just sync plaintext markdown files and use an editor on each platform. Anything else and you'll just eventually end up in the same situation.

Anyone else tried Acreom? I don't think it's open source though, but a local first markdown editor with some cool features like integration with Jira and has a nice UI.

That said, I still haven't decided if I will stick with it. I use Obsidian for journaling, brain dumps, and documenting projects in working on. Ive tried also using it as a Google Keep replacement, but its not really geared for the To-do/shopping lists that I use Keep for.

I've used vim with a smattering of essential plugins for years to do this, and only this year moved to Neovim for the same.

It's not Open Source, but I've also taken a hefty liking to Obsidian's canvas mode. Likewise, I share a small selection of lists with my other half via Google Keep.

Turtl

I have been using QOwnNotes for about 6 months. It is cross platform, lightweight, extensible and a plain-text markdown note-taking program written in C++/Qt. It can integrate with Nextcloud. Installable via scoop on Window and apt on Debian† (after installing their apt key).

Author is quite responsive on GH issues.

† Also most major Unixes

Affine.pro has potential but a couple months from cloud.

SiYuan is solid and FOSS (GPL)

I personally use Markor on Android and (neo)vim on Linux. I sync the note files using rsync (but syncthing is totally viable also).

It's probably not the solution you want but thought I'd mention it for others interested.

Depends on what method h use for note taking.

If it's not that professionally or for research purposes used then simple note is great.

If it's about research data then obsidian.

Also u will obviously find it difficult to find a good FOSS app for note taking that many of them won't support sync and collaboration.

I've been using silverbullet.md but it's self hosted

Emacs + org-mode + maybe org-roam

Amazing features, can export to a bunch of common formats.