My Nix configuration files
Has a .nix file per machine.
ln -s path/to/nix-config/machines/<machine> /etc/nixos/configuration.nix
Has a .nix file per user.
ln -s path/to/nix-config/users/<user> /home/<user>/.nixpkgs/config.nix
Has expressions with package collections.
Has expressions that can be used with nix-shell.