I have always enjoyed trying to gain access to things I'm not really supposed to play around with. I found Hack This Site a long time ago and I learned a lot from it. The issue I have with HTS is that they haven't updated their content in a very long time and the challenges are very similar. I'm no longer 13 and I want bigger and more complex challenges.
I was thinking about chalanges like Cyber Security Challenge and US Cyber Challenge (@sjp wrote about these on the meta)
Also, are there any big social engineering competitions besides the one at Def Con?
Curent list:
Wargames:
Over The Wire They have lots of small hacking challenges like: analyze the code, simple tcp communication application, crypto cracking.
We Chall We Chall is similar to Over The Wire. Lots of challenges. They also have a large list of other sites with similar challenges.
Smash The Stack
spider.io
Downloads:
Damn Vulnerable Web Application
Google Jarlsberg
exploit Exercises
Competitions:
DC3 Is the DOD:s Forensic Challenge. It's an annual competition with different scenarios that you gain points for solving.
NetWars Offers tournaments at some conferences. And one longer challenge over 4 months. With scenario challenges.
Cyber Security Challenge Needs description
US Cyber Challenge Needs description
Codegate Quals
CRT:
iCTF iCTF is a capture the flag contest held once a year.
PlaidCTF
Hack.lu CTF 2012
Defcon Quals
RuCTFe
Other list like this one:
http://captf.com/practice-ctf/
http://www.stumbleupon.com/su/1YNSxi/www.brighthub.com/internet/security-privacy/articles/77093.aspx/
http://www.wechall.net/challs
Other interesting sites:
http://captf.com/, calendar
http://facebook.com/hackercup
Please help me add more to the list.