Microsoft has announced its non-windows OS, Singularity version 1 at its Microsoft Research TechFest 2008 event in Redmond, Wash on 4th March 08.

Singularity is not based on Windows; it was written from scratch as a proof-of-concept. This Singularity OS and set of related tools and libraries that is developed completely in managed code.

Singularity was written in C#, and its easier to analyze than other non-managed operating systems. Singularity also demonstrates that software-isolated processes (SIPs) result in less performance overhead than the hardware-protection schemes commonly used by most operating systems. Singularity’s developers believe that Singularity can result in 30 percent savings in performance.
Singularity Version 2 — will focus primarily on heterogenous, multicore computing environments.

You can download Singularity Version 1 @ http://www.codeplex.com/singularity