Government uses a different OS (Score 5)

The government doesn't use Windows, Linux or xBSD for its truly sensitive documents. Instead, the DoD uses Wang's XTS-300, which is tested more extensively than the OpenBSD project and is the highest security rated operating system in existence, as seen here. One thing I thought was cool about this system is that you can't tell with 100% certainty disk space because users could in theory devise a scheme where they could pass messages encoded in changes in availability. For the same reason, if you time a process, some margin is added to the value you would get, which makes message passing take extremely long. The full specs of the Common Critera, an updated "Orangebook" are here.