Security-Enhanced Linux
From the site:
As part of its Information Assurance mission, the National Security Agency has long been involved with the computer security research community in investigating a wide range of computer security topics including operating system security. Recognizing the critical role of operating system security mechanisms in supporting security at higher levels, researchers from NSA’s Information Assurance Research Group have been investigating an architecture that can provide the necessary security functionality in a manner that can meet the security needs of a wide range of computing environments.
Thanks Hans