Microsoft research labs put a distribution and the source of Singularity (http://research.microsoft.com/os/singularity/) online today. 

Singularity is an academic project to produce a micro-kernel based managed operating system and makes for decent reading.  They're trying to produce an operating system with as much of the core written in managed languages as possible, and the source is definitely interesting, if somewhat intimidating, to poke around in.  Purely academic for the moment, as it's the sort of technology that'll probably not see use in mainstream operating systems / Windows for probably about half a decade yet, but it's quite nice to see a bootable sample and source laying the foundations of a secure operating system that isn't the quagmire presented to us by modern *nix clones.

An interesting academic footnote more than anything, but it's definitely cool.