BackTrack 5 is awesome.

BackTrack 5 is a Linux based operating system that comes pre installed with a numerous amount of security programs on it. It doesn't matter if you are a beginner or an expert you will find several tools that are installed on it that will help you to achieve your goals.
There are several different aspects of the Backtrack operating system that allow it to be the tool of choice for security professionals. One of these is the amount of different categories of security that the operating system maintains software for.
These categories include information gathering, network mapping, vulnerability identification, web application analysis, Digital forensics, reverse engineering, and basic penetration.
 We’ll start with BackTrack. The open source community has been the heart and soul behind its growth from a personal side project to over 5 million downloads.  As times go on it gets harder to devote the time and effort needed to maintain, update and enhance BackTrack.  That is no longer a problem as a funded team is now arranged to ensure those things get done.  We now have a roadmap that lands you at the end with the release of BackTrack 5.

You can get Backtrack 5 here.