Backtrack is a well-known Linux distribution that includes security tools for network penetration testing , forensics , reverse engineering and much more.It comes in 32 and 64 bits and you can get it under either Gnome or KDE . It's completely free as are all Linux distributions.You can check the avaiable distributions here:
http://www.backtrack-linux.org/downloads/

I'll be using the latest version Backtrack 5 R2 for my upcoming tutorials.