grsecurity is a collection of security patches for the Linux kernel. It enhances the security functionality of the kernel including adding a Role-Based Access Control system. It also adds a number of protections against common kernel-based attacks, enhances auditing capabilities on the system and adds considerable security to the chroot function that allows you to run certain processes in a "chroot jail."
I recommend reading the extensive Features section of the grsecurity web page at http://www.grsecurity.net/features.php. There is also a Wiki at http://www.grsecurity.net/wiki/index.php.
This was first published in January 2007