Hacking challenges

http://captf.com/practice-ctf - practice CTF list

http://www.thisislegal.com - recommended for beginners

https://hack.me

https://www.pentesterlab.com

https://www.hacking-lab.com/index.html

https://ctf.csaw.io/challenges

http://www.wechall.net - the biggest international challenges site

http://canyouhack.it

http://www.wixxerd.com

http://www.bqbi.net

http://www.microcontest.com/?lang=en

http://www.bright-shadows.net - one of the best, 341 levels

http://www.caesum.com - some of the hardest maths, crypto and cracking challenges

http://www.3564020356.org - 28 challenges about reversing

http://overthewire.org/wargames

http://www.mathchall.com/en

http://www.dareyourmind.net

https://hacking.allowed.org/cgi-bin/pageview.cgi?page=news

http://halls-of-valhalla.org/beta/challenges

https://www.root-me.org/?lang=en

http://www.securitytraps.pl

http://chall.tasteless.eu

http://www.hacker-project.com

http://hax.tor.hu

http://www.hackthissite.org

https://www.hellboundhackers.org

https://www.enigmagroup.org

http://www.rankk.org

http://www.javaist.com/rosecode

http://www.lost-chall.org - challenges based on LOST movie series, 180 levels

http://www.trythis0ne.com

http://www.hacker.org

https://www.mibs-challenges.de

https://www.net-force.nl

http://www.try2hack.nl

http://www.slavehack.com

http://www.osix.net - returning soon - server move

https://www.hackthis.co.uk

http://overthewire.org/wargames

http://www.pythonchallenge.com

https://ringzer0team.com

http://challengeland.co

http://webhacking.kr

http://pwnable.kr

http://reversing.kr - cracking and reversing challenges

http://solveme.kr - rev, cryptop, web, coding and other challenges

http://codeshell.kr

http://www.wargame.kr

http://mirror.otp22.com/chal

https://chall.stypr.com

http://www.mod-x.co.uk/main.php

https://gekko.csokavar.hu/login.php

https://www.sabrefilms.co.uk/revolutionelite

https://w3challs.com

http://www.dvwa.co.uk - Damn Vulnerable Web Application, PHP/MySQL web application

http://www.gameofhacks.com - SQL injection challenges

https://redtiger.labs.overthewire.org - SQL injection challenges

http://zixem.altervista.org - SQL injection challenges

http://google-gruyere.appspot.com - Web Application Exploits and Defenses

https://xss-game.appspot.com - Google XSS challenge game