Network Security Software

There is a boot cd somewhere that will boot your computer to a Linux OS so you can check network and wireless security. I have used it in the pass and it works great, but I have since lost it or miss placed it. The linux OS has alot of utilities on it to use. Does anyone know where I can find it? I have looked everywhere, but I can not remember wher I downloaded it from. It was free software.

Thanks in advance.
Russ