Offensive Security's penetration testing Linux distribution.
Kali Linux, formerly BackTrack Linux, is a Linux distribution made for the purpose of penetration testing. The tag-line of the software is "The quieter you become, the more you are able to hear".
The distribution contains a great many security tools like the Metasploit pentesting framework, port scanners, traffic analysis tools, and password crackers.
It is bootable from a CD, DVD or USB storage.
Official site: Kali.org
Official site of BackTrack (no longer maintained): backtrack-linux.org
