The National Security Agency (NSA)
Researchers in NSA's National Information Assurance Research Laboratory (NIARL) designed and implemented flexible mandatory access controls in the major subsystems of the Linux kernel and implemented the new operating system components provided by the Flask architecture, namely the security server and the access vector cache. The NSA researchers reworked the LSM-based SELinux for inclusion in Linux 2.6. NSA has also led the development of similar controls for the X Window System (XACE/XSELinux) and for Xen (XSM/Flask).
SELinux Project Wiki
The UnOfficial SELinux FAQ