Learning to Hack - Vulnerable Testbeds

There are a crap ton of vulnerable testbeds to educate the interested in how applications/operatings systems get hacked. I'll update this list as I come across them:

http://vulnhub.com/
http://io.smashthestack.org:84/
https://github.com/stripe-ctf/stripe-ctf
https://github.com/stripe-ctf/stripe-ctf-2.0/
http://www.dvwa.co.uk/
http://www.offensive-security.com/metasploit-unleashed/Metasploitable
http://www.irongeek.com/i.php?page=mutillidae/mutillidae-deliberately-vulnerable-php-owasp-top-10
https://github.com/SpiderLabs/SQLol
https://github.com/SpiderLabs/ShelLOL
https://github.com/SpiderLabs/XMLmao
https://github.com/SpiderLabs/XSSmh
https://github.com/SpiderLabs/CryptOMG
https://www.pentesterlab.com/exercises
http://www.overthewire.org/wargames/

EDIT:

Recently found these links on reddit for Capture The Flag challenges:

https://github.com/isislab/Project-Ideas/wiki/Capture-The-Flag-Competitions