HellBoundHackers is a completely legal, web-based security training ground. They offer challenges that teach you how computer based exploits work. The idea being, if you know how to exploit a website for instance, then you can go and secure your website, and help others in securing theirs. If you know how malicious hackers get in, you can keep them out. HBH also supplies a vast library of downloads, articles, and helpful forum posts.