The Raspberry Pi is a mini computer, so you can run a number of Linux security tools using a Pi. Roll your own, or try out one of the pre-built images below.
Kali Linux Kali (Formerly Backtrack) is a popular Linux pentesting distro (distribution) with releases available for the Raspberry Pi. This is a good place to start with pentesting. More advanced users may prefer rolling their own security distro, but this is a good place to start.
Pwnie Express – Pwnie Express is a company that makes pentesting hardware. They put out a script to setup some of the features that are available on their hardware for the Pi. It does not appear that they are maintaining this script.