Cosmos is an open source microkernel .Net-based operating system written entirely in C#. The folks over at SharpOS.org, who have also created a microkernel OS written in C#, have reasonably objected to claims that Cosmos is the first OS of this type. Both operating systems can be installed and run from a USB jump drive, which only adds to their geeky cred.

You can get updates to the Cosmos project at the official blog. A Cosmos FAQ is also available at the site.