I would not call these projects unbelievable, but they are neat.

Opt-in state:

https://github.com/nix-community/impermanence

https://grahamc.com/blog/erase-your-darlings

Ease of setting up a real-time audio on Linux:

https://github.com/musnix/musnix

Generating virtual machines/installators from a configuration:

https://github.com/astro/microvm.nix

https://github.com/nix-community/nixos-generators

It's interesting to scroll through nix-darwin options, I wasn't even aware of some useful macOS options before:

https://daiderd.com/nix-darwin/manual/index.html

A neat collection of music information retrieval packages in one place (and the ease of creating your own package registry):

https://github.com/carlthome/mirpkgs