Penetration testing is a method to enter into a network or computer without the knowledge of user-name and password, in short penetration testing means to check the IT infrastructure for various vulnerability that an attacker may use to gain access into the network.
The person who do this test known as Pen-tester, normally penetration testing is done by Linux distribution with a multiple tools. Now there are many Distro that can perform penetration testing with the powerful tools, which one is the best is depend on the tester or the test that going to be held.
BackTrack is a most famous and widely used Pen-tester distribution out there.The evolution of BackTrack spans many years of development, penetration tests, and unprecedented help from the security community. BackTrack originally started with earlier versions of live Linux distributions called Whoppix, IWHAX, and Auditor. BackTrack 4 R2 is the latest version that has been released, it has more than 300 security tools to perform pen-testing.
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. This distribution did not get any update in a long time but still a useful distro.
Network Security Toolkit
Network Security Toolkit (NST). This bootable ISO live CD/DVD (NST Live) is based on Fedora. The toolkit was designed to provide easy access to best-of-breed Open Source Network Security Applications and should run on most x86/x86_64 platforms.
Peento is a smart and well equipped linux distro with various tools, based on Gentoo.It's basically a gentoo install with lots of customized tools, customized kernel, and much more.