Quite recently, the GNU project has announced Guix, a new package manager for the GNU system. Guix is described on their website as:
GNU Guix is a purely functional package manager, and associated free software distribution, for the GNU system. In addition to standard package management features, Guix supports transactional upgrades and roll-backs, unprivileged package management, per-user profiles, and garbage collection.
Read the full thing at http://sandervanderburg.blogspot.co.uk/2012/11/on-nix-and-gnu-guix.html