Profit with Nix!

gista-file

{ allowUnfree = true;  # allow packages like skype to be installed
  allowBroken = true;  # you probably want this since on osx since quite
                       # some packages are marked as not tested (aka broken)
                       # on darwin platform. packages that are marked as
                       # broken/untested will be build from source (no binary
                       # exists for them)
  allowUnfreeRedistributable = true;
                       # some packages require this option to be set
                       # explicitly. this is sometimes a way to "confirm" that
                       # you agree with terms and conditions of upstream 
                       # redistribution. since nix handles everything via
                       # config files also agreement is handled via
                       # configuration files.

  # here you specify a dictionary of packages that you want to override
  # original nixpkgs. as you can see packageOverrides expects to be a function
  # with one argument, which is a set you are extending from.
  # this is also a way how to add new packaes like is a case in example bellow.
  # we are create a new package which serves as a meta package which pulls in
  # other packages.
  # that way you only keep installing (which also means upgrading) one package
  # and that pulls in other packages.
  #    % nix-env -iA nixpkgs.all     <- on non-NixOS systems
  #    % nix-env -iA nixos.all       <- on NixOS systems
  # next time when you want to have additional package you add it to the list
  # bellow. and rerun the above command.
  packageOverrides = pkgs: {
    all = pkgs.buildEnv {
      name = "all";
      paths = with pkgs; [
        firefox-beta-bin
        git
        neovim
      ];
    };
  };

  # also dont forget to put this file under VCS control :) and profit!
}