From Exploit-Exercises.com:
exploit-exercises.com provides a variety of virtual machines, documentation and challenges that can be used to learn about a variety of computer security issues such as privilege escalation, vulnerability analysis, exploit development, debugging, reverse engineering, and general cyber security issues.
I like the idea of having a safe playground to do hands on security experiments. Reminds me of the Stripe CTF ( Capture The Flag ) events, which were really well done.