The latest stable release series is 14.04. Below are links to CD/DVD images and VirtualBox appliances containing the latest release in this series.
The graphical installation CD contains the NixOS installer as well as X11, KDE 4 and several applications. It’s a live CD, so it allows you to get an impression of NixOS (and the Nix package manager) before installing it.
The minimal installation CD does not contain X11, and is therefore a lot smaller. You have to run the installer from the console. It contains a number of rescue tools.
This is a demo appliance for VirtualBox (in OVA format) that has X11 and KDE enabled, as well as the VirtualBox guest additions. To use it, download the OVA file, open VirtualBox, run “File → Import Appliance” from the menu, select the OVA file, and click “Import”. You can then start the virtual machine. When the KDE login screen appears, you can log in as user demo, password demo. To obtain a root shell, run sudo -i in the KDE terminal (konsole).
If you are an EC2 user, you can fire up a NixOS instance instantly by using one of the AMIs listed below.
For instance, to create a NixOS instance in region ap-southeast-1 using the EC2 API tools, just do:
$ ec2-run-instances ami-1874294a --region ap-southeast-1 -k my-key-pair
If you are a developer who wants to contribute to NixOS, or if you just want the latest and greatest, you can also install the most recent unstable release.