Nixpkgs is free software, released under a permissive MIT/X11 license.
Note: if you are a NixOS user, you already have Nixpkgs. You can update your copy of Nixpkgs to the latest version by running nix-channel --update.
The usual way to obtain Nixpkgs is to subscribe to the Nixpkgs channel, which always contains the latest version of Nixpkgs:
$ nix-channel --add https://nixos.org/channels/nixpkgs-unstable $ nix-channel --update
After this, you can use nix-env to install available packages.
If you want a source tarball, you can download one from our continuous build system:
All releases of Nixpkgs can be found on GitHub.