There are apps like https://offpunk.net/ which are explicitly low tech (and even solar punk). But there are also apps, which just happen to be offline first (think PWA etc.). And things in between, like Syncthing. Some might be self hosted other maybe local or distributed applications.
What’s your favorite offline-first app or tool?
Can you explain what you mean by offline-first?
Like if it’s internet stuff that still works when offline, most e-mail clients would count. I use KMail but they’re all pretty similar.
Maybe I misunderstood but what OP means (that I understood) are softwares that you don’t need to rely on internet connection to use it, probably ones you need 0 need at all of internet connection but I assume OP may also be accepting suggestion of ones you can open while without internet connection but also be able to download data for offline use (maybe like Language Transfer).
Offline-first can be a lot, from my perspective. Just as yours and David’s example, an application that work without active internet connection is valid. But this is rather common and the classic way how applications just operated for a long time (despite the modern mobile apps, which have to pull some strings nowadays to allow offline access).
What I find more interesting is when a system or application overcomes the otherwise required network connection. So maybe offpunk (from above) is an example, A web browser which puts everything in a cache, so you can continue to read, if offline. But that’s a rather simple approach as well. The whole “opportunistic syncing” approach by Syncthing is also matching. It also covers some ground regarding resiliency. So another aspect could be resilience, meaning a connected system will not just survive a temporary outage of a node, but also continues to work as expected. Many distributed tools would probably count, even classics like git. ActivityPub based tools and the fediverse, on the other hand, are somewhat resilient, but not offline-first. Scuttlebutt would be though: https://scuttlebutt.nz/
Back to your examples: I think if a “simple local application” would do something we usually use network for, but technically don’t have to, this would also be interesting for me. An actual example might be: https://devtoys.app/
I got recently inspired by this, which also has its own definition of offline-first: https://gemini.tildeverse.org/?gemini://zaibatsu.circumlunar.space/~solderpunk/gemlog/announcing-offlfirsoch-2025.gmi (proxied from the Gemspace)
I do get the appeal of things needing to work without internet, but it seems very broad as a category. People use webapps for things that used to be local, like Office 365 or Figma, or even searching in Google to do arithmetic, so the calculator app is offline first.
On mobile I think a more reasonable example is offline maps, I use OSMand myself but recommend Organic Maps to less technical people.
About offpunk, all browsers used to have that. Firefox still has the “work offline” option in the file menu. In offline mode you can go to any webpage that you visited while online.
Thank you for clarifying
Ceno does that but it’s for mobile