Free and Open Source Software
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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Nextcloud Tasks?
Nextcloud is nice since iPhone natively supports caldav/carddav so you can use the built in reminders app.
Joplin may be an option
I love joplin, its so simple and intuitive. I also use it to produce simple manuals and documentation for my code.
It’s the best option I’ve found. Plus the fact that it’s easy to synchronize between iOS, windows and Linux is amazing.
Vikunja (hosted on tchncs: https://todo.tchncs.de/) is great for tasks but I haven't tried using it for a grocery list.
Vikunja is very slick!
do birds fly? do ducks duck?
I don't know what features you're looking for but I use nextcloud notes. It's really great for my uses, I keep categorized shopping lists, to do lists, email drafts, etc all in one app. I can access it trough the Android app on my phone and through the web frontend on my nextcloud instance from anywhere.
I use carnet off of fdroid
I've been trying out Carnet for a few days, syncs with Nextcloud and has an app for the dashboard and a mobile app for android.
So far I like it, a little clunky in some ways, but overall solid and looks nice.
You can just use plain todo.txt. I wrote an extension for it as well, but it's a bit niche for a certain type of users: https://proycon.anaproy.nl/posts/todo/
I don't think it's open source but we use our groceries. Just links to an email only no password. They just send you a code when you link the email to a new app. It's also cross platform if that matters at all. Also requires no permissions granted to run and only requests a camera permission to scan barcodes.
Orgzly app for android. It is based on org-mode plain text files and support sync via WebDav. Org-mode is one of the oldest format of todo files and have a huge community/plugins/guides/etc.
Because it is plain text in very popular format it may be easily migrated between different cloud storages and may be edited from PC tooo in a any text editor.
Also format of org files is human readable.