Nix
A package manager built on the idea that an installation is a pure function of its inputs. Came out of Eelco Dolstra's doctoral work (2003–2006). The foundation NixOS stands on.
Every package is built in isolation and lands in /nix/store under a path containing a hash of everything that affected the build — source, compiler version, flags, dependencies. Two packages needing different versions of the same library can therefore coexist without conflict, upgrades are atomic, and rollback is just repointing a symlink. It solves "works on my machine" for real, and nixpkgs is one of the largest package repositories in the world. The downside is the learning curve: a bespoke lazy functional language, documentation that has long been notorious, and a transition to flakes that has been experimental for years. The community also had a bitter governance conflict in 2024.