Here's a mixture of applications, some for Linux, some for Android, some for both. And some of them might work on other platforms as well, but I'm not sure.
Borg for making backups. For the first glance it could look overwhelming, but after reading through the quick start guide, it's really easy to use.
VeraCrypt for encryption of removable media.
Megalodon as a Mastodon client.
Voyager as a Lemmy client. It has a very weird and unintuitive UI, but there are no ads and the content is well readable, well presented.
OsmAnd for offline navigation. It's especially great for cycling and hiking, as even the most insignificant trails are on the map. It isn't free, but it's cheap.
Thunderbird for emails. Until recently I just used the online interface for my emails, but ever since I got a Proton subscription and multiple aliases with it, I started to use Thunderbird so I can see everything in one place, and also it has advanced filtering capabilities (the best of any email client I've ever used).
Proton Calendar, just for the sake of not to use Google.
Firefox with uBlock Origin. These two together is the bare minimum nowadays if you are thinking about browsing the internet.
VS Code for smaller stuff. Not gonna list my extensions here, but there are a few less known ones that I always install.
Zed is in early development, but if it gets as mature as VS Code, I'll consider using this instead.
JetBrains IDEs for software development. It makes me cry every year when I spend a buttload of money on renewing my license, but for me it's worth it. No other IDE ever made it so easy for me to set up and work with projects.
Dia for UML or database schema diagrams, and bunch of others. Sadly it's a bit outdated, but it's simple and easy to use.