Much publicity exists around NSA's open-source Security Enhanced Linux. People have discussed it in articles, at various conferences, in presentations and user group meetings. I made a few attempts in the early days to have it work on a server or two myself. After many frustrations, the closest I ever got to a working version was Mark Westerman's laptop at the Boston University Linux Desktop Conference in November 2003.
Security Enhanced Linux enforces a mandatory access control model of security. This differs from the discretionary model used in UNIX and Linux operating system. SE Linux policies confine user programs and system services to the least privilege they require to do their jobs.