The following websites offer you a free and legal way to acquire practical hacking skills. If you are going to stop the bad guys, you need to know how they act to protect your own servers and wargames are the best way to be one of the bad guys without worrying about the FBI knocking at your door or harming anybody.
The computers you will be hacking in wargames are virtual machines that can be easily reset, and if you get lost, a community of white hackers will be willing to help you out teaming up with you or sharing experiences.
Exploit Exercises: A site giving you access to various virtual machines to hack, you will be given challenges, like scanning a network to find what vulnerabilities exist and how to exploit them. This site is admirably structured with the servers separated in between different hacking skills and levels. You can download a .iso or .ova (Open Virtual Application) and run it locally in your computer to hack it as if it was online.
Hacking Lab: An IT security portal with various hacking tests, it has its own custom live CD with a VPN connection that you can use for hacking. Just like in real life, where you have to scan a server to fingerprint them before launching an exploit, in hacking lab you will have to find the IP or DNS of the vulnerable server before a hacking attack can take place.
Pen Tester Lab: Full of penetration training exercises for people interested in becoming a PEN tester. You are given weekly computer security exercises in the boot camp section, lessons will get more and more difficult as you complete them. Tasks are clearly explained with links to the files you have to download if necessary.
HackThisSite: One of the wargame sites that has been the longest around, with a great hacking community that will help you expand your skills, you can chat with like minded people in HackThisSite forums or in the old school hacker’s communication tool IRC. This site stands out from the crowd with their extensive amount of free learning resources.
Hacker Project: A fictional hacking game set in the future where governments have gone bankrupt and multinationals take over the World stopping free flow of information. Your job will be to return power to the people by infiltrating corporations and use their information technology network against them. This site is for entertainment, you don’t hack anything for real, but the game is realistic.
HackerForEver: Text based browser game revolving around the dark world of hackers, from the good white hackers up to the bad black hacker guys. You can choose what side you would like to be on, the game has various clans you can join and a community. You will not do real hacking here, just a simulation, games like this serve as introduction to the hacking slang world, suitable for people of any level.