Over the last little while, I've fallen down the Nix rabbit hole.

I don't currently use it in any serious projects aside from tinkering about with it, but it has been a lot of fun to learn and study.

Between the Nix package manager, the associated language, etc., there has been a lot to learn about, and it's been good fun. I have nixOS on my spare Thinkpad for toying with, and I have Nix on my main Debian systems, if I want to pull something from nixpkgs.