GNU Privacy Guard
   OpenPGP (RFC2440) implementation from the GNU project. Encrypts/decrypts/signs/digests files using strong cryptography. FAQ, downloads, documentation.
   http://www.gnupg.org/

LBNL's Network Research Group
   Tools such as tcpdump and traceroute.
   http://www-nrg.ee.lbl.gov/nrg.html

LogSurfer
   Allows improved monitoring and alerting based on the contents of system logs.
   http://www.dfn-cert.de/eng/logsurf/

NMAP Network Scanner
   Efficient stealth port scanner. Downloads, e-mail lists, news.
   http://www.nmap.org/

North American Cryptography Archives
   Strong cryptographic software, libraries, and information about cryptography, data security, and privacy.
   http://cryptography.org

OpenSSH - Portable Distribution
   Open source implementation of SSH1 and SSH2 protocols. Ported from OpenBSD by the OpenSSH portability team.
   http://www.openssh.org/portable.html

OpenSSL
   The OpenSSL Project is a collaborative effort to develop a robust, commercial-grade, fully featured, and Open Source toolkit implementing the Secure Sockets Layer and Transport Layer Security protocols with full-strength cryptography world-wide.
   http://www.openssl.org/

Pidentd
   Daemon implementing IDENT protocol, it can be used to identify the user who initiated a TCP/IP connection
   ftp://ftp.lysator.liu.se/pub/ident/servers/

Postfix Mail Transfer Agent
   Drop-in Sendmail replacement by Wietse Venema. Offers several features that make it difficult to crack.
   http://www.postfix.org

S4Software
   Use Secure4Privilege for complete access control to commands and scripts without giving out the root password.
   http://www.s4software.com/sg.htm

SAINT
   (Security Administrator's Integrated Network Tool) Scans systems and networks and reports potential vulnerabilities.
   http://www.wwdsi.com/saint/

Sudo
   Run selected commands as root, with the users' own password. Allows the administrator to grant limited root privileges to a subset of users, without giving out the root password.
   http://www.sudo.ws/

Symark Software
   Execute root-level commands and shell scripts without having to actually login as root. Control which users can log in and under which circumstances.
   http://www.symark.com/products.htm

The Coroner's Toolkit (TCT)
   The Coroner's Toolkit (TCT) provides post-break-in data collection that could be useful in determining what happened. Less polished that the authors (Dan Farmer and Wietse Venema) usual work, the toolkit offers a patch-work of tools that help exhume
   http://www.porcupine.org/forensics/tct.html

The SUS Home Page
   SUS is a utility to allow a user (typically a system administrator) to run a single command as the super user.
   http://pdg.uow.edu.au/sus/

Tripwire Inc.
   Commercial version of the original Unix file integrity scanner. Calculates and stores signatures of file permissions, ownership and contents. Scans the same files later, detecting changes.
   http://www.tripwire.com/

Tripwire Open Source
   Open source version of the original Unix file integrity scanner. Calculates and stores signatures of file permissions, ownership and contents. Scans the same files later, detecting changes. This open source version is targeted at Linux systems.
   http://www.tripwire.org

lsof
   lsof is a tool for examining I/O channels open on your Unix system. These can include files, FIFOs and network sockets. This information can be extremely useful for security and a variety of other purposes. Works on most Unix-like systems.
   http://freshmeat.net/projects/lsof

mcrypt
   A replacement for the unix crypt, using several block algorithms in several modes like cbc, and cfb.
   http://hq.hellug.gr/~mcrypt

sudoscript
   A pair of Perl scripts that provide an audited root shell using sudo and script(1). Tarballs, RPM's and individual components.
   http://www.egbok.com/sudoscript