NixOS 14.12 “Caterpillar” has been released, the third stable release branch. It brings Linux 3.14, systemd 217, Glibc 2.20, KDE 4.14.1, and much more. See the release notes for details. You can get NixOS 14.12 ISOs and VirtualBox appliances from the download page. For information on how to upgrade from older release branches to 14.12, check out the manual section on upgrading.