Security Features

Firewall Tools (from the Application Management section)
Server Virtualization (from the System Management section)
GRsecurity (Collection of popular security patches that will apply cleanly to several recent stable kernel versions) (cvs) (ml) (forums)

LSM ("Linux Security Module". Kernel interface for developing and using different security-policy related plugins) (source) (ml) (RSBAC comments on LSM) (GRsecurity comments on LSM)
PAM for Linux ("Pluggable Authentication Modules") (PAM Modules and Applications) (ml)
Bestbit ACL's (Extended attributes and access control list framework, with support for various file systems) (ml)
RSBAC ("Rule Set Based Access Control". Mandatory access control lists system.) (ml)
SE Linux ("Security-Enhanced Linux". (LSM compatible, mandatory access control lists system, developed by the NSA.) (ml) (serefpolicy alternative to the existing strict and targeted policies) (Debian SE Linux)
Umbrella (LSM compatible mandatory access control lists system, including executable signing, for embedded Linux systems) (cvs) (ml)
Port pseudo ACLs (Kernel patches which allow an administrator to delegate privilege for some protected network resources to non-root users mechanisms for Linux)
Linux Trustees (LSM compatible access control lists system)

The GNU/Linux CryptoAPI (Linux kernel interface to allow for the encryption/decryption of all physical media and network traffic) (cvs) (ml) (IKP as pure module)
Linux Crypto Mailinglist (Discussion list for cryptography related Linux topics)
strongSwan (IPsec implementation. StrongSwan is a code-fork of the inactive FreeS/WAN project.) (ml)
Cryptoswap (Kernel patches for building an encrypted swap partition when a system boots)