NixOS
 

"In NixOS, the entire operating system - the kernel, applications, system packages, configuration files, and so on - is built by the Nix package manager from a description in a purely functional build language. The fact that it's purely functional essentially means that building a new configuration cannot overwrite previous configurations. Most of the other features follow from this." Interesting approach. A Linux distribution, sure, but with some very refreshing ideas about system configuration.

http://osnews.com/story/27047/NixOS