GnackTrack

GnackTrack is a Live (and installable) Linux distribution designed for Penetration Testing and is based on Ubuntu. Although this sounds like BackTrack, it is most certainly not; it's very similar but based on the much loved GNOME!
Download GnackTrack

MatriuX

The Matriux is a phenomenon that was waiting to happen. It is a fully featured security distribution consisting of a bunch of powerful, open source and free tools that can be used for various purposes including, but not limited to, penetration testing, ethical hacking, system and network administration, cyber forensics investigations, security testing, vulnerability analysis, and much more. It is a distribution designed for security enthusiasts and professionals, although it can be used normally as your default desktop system.
With Matriux, you can turn any system into a powerful penetration testing toolkit, without having to install any software into your hardisk. Matriux is designed to run from a Live environment like a CD / DVD or USB stick or it can easily be installed to your hard disk in a few steps. Matriux also includes a set of computer forensics and data recovery tools that can be used for forensic analysis and investigations and data retrieval.
Download MatriuX

BackTrack

BackTrack 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. Regardless if you’re making BackTrack your primary operating system, booting from a LiveDVD, or using your favorite thumbdrive, BackTrack has been customized down to every package, kernel configuration, script and patch solely for the purpose of the penetration tester.
BackTrack is intended for all audiences from the most savvy security professionals to early newcomers to the information security field. BackTrack promotes a quick and easy way to find and update the largest database of security tools collection to-date. Our community of users range from skilled penetration testers in the information security field, government entities, information technology, security enthusiasts, and individuals new to the security community. Feedback from all industries and skill levels allows us to truly develop a solution that is tailored towards everyone and far exceeds anything ever developed both commercially and freely available. The BackTrack Project is funded by Offensive Security. Download BackTrack

Pentoo

Pentoo is a security-focused livecd based on Gentoo
It's basically a gentoo install with lots of customized tools, customized kernel, and much more. Here is a non-exhaustive list of the features currently included :

• Kernel 2.6.31.6 with lzma and aufs patches
• Wifi stack 2.6.32_rc7
• Module loading support ala slax
• Changes saving on usb stick
• Enlightenment DR17 WM
• Cuda/OPENCL cracking support with development tools
• System updates if you got it finally installed

Download Pentoo

S-T-D 0.1

STD is a Linux-based Security Tool. Actually, it is a collection of hundreds if not thousands of open source security tools. It's a Live Linux Distro, which means it runs from a bootable CD in memory without changing the native operating system of the host computer. Its sole purpose in life is to put as many security tools at your disposal with as slick an interface as it can.
Download S-T-D