Underappreciated FOSS projects

Nix/NixOS: Functional programming language, package manager, build system, software distribution system, system configuration system, and probably a lot more.
FreeTube: YouTube frontend without tracking or advertising. Also check out Invidious and NewPipe.
/e/ OS: de-Googled Android/LineageOS fork focused on privacy and ease of use. Absolutely stable and also suitable for non-techies. Also sells smartphones with pre-installed /e/.
Termux: Complete Debian-like GNU/Linux subsystem for Android that doesn’t require rooting your phone.
Qalculate!: Multi-purpose calculator application with support for physical units, so you can type expressions like (1m + 2cm) / 1 h to m/s. Incredibly useful in experimental physics.
Xournal++: Notetaking app with support for touchscreens and graphics tablets.

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