Backtrack

BackTrack is a distribution based on the Debian GNU/Linux distribution aimed at digital forensics and penetration testing use.It is named after backtracking, a search algorithm

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BackTrack provides users with easy access to a comprehensive and large collection of security-related tools ranging from port scanners to password crackers. Support for Live CD and Live USB functionality allows users to boot BackTrack directly from portable media without requiring installation, though permanent installation to hard disk is also an option.

BackTrack includes many well known security tools including:
Metasploit for integration
RFMON, injection capable wireless drivers
Aircrack-ng
Gerix Wifi Cracker
Kismet
Nmap
Ophcrack
Ettercap
Wireshark (formerly known as Ethereal)
BeEF (Browser Exploitation Framework)
Hydra
OWASP Mantra Security Framework, a collection of hacking tools, add-ons and scripts based on Firefox
Cisco OCS Mass Scanner, a very reliable and fast scanner for Cisco routers with telnet and enabling of a default password.

A large collection of exploits as well as more commonplace software such as browsers.
BackTrack arranges tools into 12 categories:
Information gathering
Vulnerability assessment
Exploitation tools
Privilege escalation
Maintaining access
Reverse engineering
RFID tools
Stress testing
Forensics
Reporting tools
Services
Miscellaneous