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

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

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

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/

GASP! -- Georgetown Automated Solaris Patchtool  
   Automate Sun's Patchdiag tool with a command-line or X-Windows interface.
   http://www.georgetown.edu/reillyb/gasp

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

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/

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/

Virtual eXecuting Environment  
   VXE defines restrictions for filesystem and all syscalls. More convenient than chroot(); (free for non-commercial use).
   http://www.intes.odessa.ua/vxe

Netpassword  
   Changing passwords safely across the net.
   http://www.brunel.ac.uk/depts/cc/papers/netpassword-paper/paper.html

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

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

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/

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

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

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

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

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

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/

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

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