SELinux

Security Enhanced Linux (SELinux) is a security enhancement to Linux implementing mandatory access control (MAC). Users and processes can be granted their least required privileges in a much more granular way than with traditional Unix access control. For example, you can define a policy to prevent your web browser from reading your SSH keys. The security model of SELinux has been ported to other operating systems; see SEBSD for FreeBSD and Project fmac for OpenSolaris.

Knoppix

Knoppix consists of a representative collection of GNU/Linux software, automatic hardware detection, and support for many graphics cards, sound cards, SCSI and USB devices and other peripherals. Knoppix can be used as a productive Linux system for the desktop, educational CD, rescue system, or as many Nmap survey takers attest, a portable security tool.

Samurai Web Testing Framework

The Samurai Web Testing Framework is a live linux environment that has been pre-configured to function as a web pen-testing environment. The CD contains the best of the open source and free tools that focus on testing and attacking websites. Samurai includes many other tools featured in this list, such as WebScarab, ratproxy, w3af, Burp Suite, and BeEF.

Helix

Helix is a Ubuntu live CD customized for computer forensics. Helix has been designed very carefully to not touch the host computer in any way and it is forensically sound. Helix will not auto mount swap space, or auto mount any attached devices. Helix also has a special Windows autorun side for Incident Response and Forensics. Downloading of the live CD is only provided as a complement to membership in the e-fense members-only forum. An unsupported, older, no-cost version is available as well.

BackTrack 5 R3

The highest rated and acclaimed Linux security distribution to date. BackTrack 5 R3 is a Linux-based penetration testing arsenal that aids security professionals in the ability to perform assessments in a purely native environment dedicated to hacking.