Cosmos? Is out there or is it a new OS written in C#

Well, when in doubt, check out for yourself.
URL: http://gocosmos.org/

Released under BSD Open Source license

If you need to see in order to believe, check out the screenshots section:
http://gocosmos.org/Screenshots/index.aspx

From the site:
"Why Cosmos?
Firstly because its fun! But there are many scenarios to use Cosmos in real world applications."

"Get source from CodePlex"