Test Your Hacking Skils

http://modx.co.uk://http://modx.co.uk
http://www.try2hack.nl/://http://www.try2hack.nl/
http://www.bright-shadows.net://http://www.bright-shadows.net (best challenges)
http://www.arcanum.co.nz://http://www.arcanum.co.nz (very good variety)
http://www.hackquest.de://http://www.hackquest.de (2nd best challenges)
http://net-force.nl://http://net-force.nl
http://www.osix.net://http://www.osix.net (good programming/logic challenges)
http://www.learntohack.org://http://www.learntohack.org (easy starter)
http://www.spy-games.net://http://www.spy-games.net (down for a little bit)
http://www.pulltheplug.org://http://www.pulltheplug.org (wargame style)
http://www.slyfx.com://http://www.slyfx.com
http://www.hackerslab.org/eorg/://http://www.hackerslab.org/eorg/ (very very good linux hacking, also hard)
http://biw.rult.at/index.php://http://biw.rult.at/index.php (cracking and reverse engineering)
http://zyclonite.net/://http://zyclonite.net/ (I'm not sure, a few challenges, a lot of info I think)