NixOS 15.09 “Dingo” has been released, the fourth stable release branch. See the release notes for details. You can get NixOS 15.09 ISOs and VirtualBox appliances from the download page. For information on how to upgrade from older release branches to 15.09, check out the manual section on upgrading.
NixOS 15.09 released
Published on Fri Oct 30 2015