We have released NixOS 13.10, the first stable branch of NixOS. Its goal is to provide a safe branch for production environments that need bug fixes and security updates, but not the potentially destabilising changes that sometimes occur on the unstable branch. You can get NixOS 13.10 ISOs and VirtualBox appliances from the download page. See the announcement for more information. For information on how to switch an existing NixOS machine from the unstable channel to 13.10, check out the manual section on upgrading.