Resources

pwnable.kr 

 pwnable.kr is a non-commercial wargame site which provides various pwn challenges regarding system exploitation. the main purpose of pwnable.kr is 'fun'.    please consider each of the challenges as a game. while playing pwnable.kr, you could learn/improve system hacking skills but that shouldn't be your only purpose.
overthewire.org

The wargames offered by the OverTheWire community can help you to learn and practice security concepts in the form of fun-filled games.

smashthestack.org

This network is non-profit and provided by volunteers and funded with money out of our own pockets. So, if you want to give back to the community, have an idea for a wargame or want to help us out in some way, hit us up on IRC or email us at staff@smashthestack.org. All donations are welcome no matter how small.

microcorruption.com

You're playing "Capture The Flag". You collect points for each level you beat, working your way through steadily more complicated vulnerabilities. Most levels showcase a single kind of real-world software flaw; some levels chain a series of them together.

pwnable.tw

Pwnable.tw is a wargame site for hackers to test and expand their binary exploiting skills.