Hack This Site is a free, safe and legal training ground for hackers to test and expand their hacking skills: http://www.hackthissite.org
More than just another hacker wargames site, we are a living, breathing community with many active projects in development, with a vast selection of hacking articles and a huge forum where users can discuss hacking, network security, and just about everything. Tune in to the hacker underground and get involved with the project.