Damn Vulnerable Linux is the most complete training environment for IT security with over 500.000 downloads. It includes all tools you need ready to go. Additionally tons of training material and exercises are included. Damn Vulnerable Linux works fine under Windows, Linux and Mac OSX using any virtual machine such as VMware, Qemu or KVM. You can let it run installed natively on a standard PC or even boot it from USB.
Damn Vulnerable Linux (DVL) is everything a good Linux distribution isn't. Its developers have spent hours stuffing it with broken, ill-configured, outdated, and exploitable software that makes it vulnerable to attacks. DVL isn't built to run on your desktop - it's a learning tool for security students.
The main idea behind DVL was to build up a training system that I could use for my university lectures. My goal was to design a Linux system that was as vulnerable as possible, to teach topics such as reverse code engineering, buffer overflows, shellcode development, Web exploitation, and SQL injection.
An interesting training environment for black hats and white hats.