SELinux
Initially a project to port the work developing a mandatory access control architecture done by the National Security Agency (NSA) and the Secure Computing Corporation (SCC) on the Mach and Fluke OS's to Linux.
http://www.nsa.gov/selinux/index.cfm
A paper on why mandatory access controls are a good and likely a necessary thing can be found here: http://www.nsa.gov/selinux/papers/inevit-abs.cfm
SELinux on Debian
The selinux suite for Debian 3.0 is found in three packages: selinux, libselinux-dev and kernel-patch-2.4-lsm. The kernel patch package also contains a port of ?OpenWall along with a port of ["SELinux"].
As of 10 May 2004 in Debian/unstable the patch depends on kernel-patch-acl which in turn brings in kernel-patch-exec-shield.
Check the package change logs for information about what kernels are supported and what other patches are required. Notes on applying the patch can be found on the ?KernelPatchPackage page.
Links to pages about Debian lsm/selinux
[http://www.coker.com.au/selinux/ Russell Coker's ["SELinux"] site]
[http://mail.wirex.com/pipermail/linux-security-module/2004-February/5027.html Re: Trouble with installation of lsm kernel patch and SE linux Packages on Debian]
[http://www.jakubholy.net/unix/selinux.html SE Linux for Debian/Testing with the kernel 2.6.6] (October 2004)