Not really an app, but I’m going to add kagi.com here; it’s time for a shakeup in the search industry that actually works, and Kagi delivers.
Controversial, but Telegram is such a solid open source app in terms of UI/UX and a good middle ground between something like Discord and Signal.
Now that I’ve shared my unpopular opinions…
Standard Notes is a really solid secure note taking app.
All of the Proton AG apps and services (Proton Drive, Proton Calendar, Proton VPN, Proton Mail, SimpleLogin).
Bitwarden.
“Privacy” (while not FOSS) is a pretty great software for using unique billing information per site (which helps with fraud protection, tracking, etc).
ZeroTier is awesome for remote access (everyone seems to recommend TailScale these days, but ZeroTier is very much of the “do one thing and do it well” mindset, they’re also the “OG” FOSS encrypted VLAN solution).
Kopia I’ve been really liking for backups (great features include: the ability to clone a cloud repository of backs to a hard drive or different cloud via the software itself, a GUI, a simple CLI interface, and configurable policies to keep track of your preferences so you don’t have to)