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 interesting stuff from violated systems. Should be installed and examined before a break-in happens!

OpenPGP (RFC2440) implementation from the GNU project. Encrypts/decrypts/signs/digests files using strong cryptography. FAQ, downloads, documentation.

Tools such as tcpdump and traceroute.

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.

A replacement for the Unix crypt, using several block algorithms in several modes like cbc, and cfb.

Efficient stealth port scanner. Downloads, e-mail lists, news.

Strong cryptographic software, libraries, and information about cryptography, data security, and privacy.

Open source implementation of SSH1 and SSH2 protocols. Ported from OpenBSD by the OpenSSH portability team.

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.

Daemon implementing IDENT protocol, it can be used to identify the user who initiated a TCP/IP connection